[Bug] Batching throws error when Flask auto-reload is enabled

Hi,

I'm currently upgrading a graphene 2 project to graphene 3. The project uses `graphene-sqlalchemy` and `graphql-server[flask].` I'm noticing strange behavior when using batching while flask is in development mode, specifically when auto-reload is enabled. When auto-reload is enabled and batching is enabled, then all requests fail. Instead an `asyncio` error is thrown, `TypeError: UserConvo fields cannot be resolved. There is no current event loop in thread 'Thread-1'.` The issue only occurs when batching is enabled and flask auto reload is enabled. Disabling batching or disabling the flask auto-reload resolves the issue. The issue also only occurs with a many to many relationship that include the `back_populates` param in the relationships.

Below is a [somewhat] minimal example to reproduce the issue. The app.py, schema.py, and requirements.txt file are below.

Let me know if you need more info or clarification. Thanks!

Repro:
0) set up a virtual env and install packages from `requirements.txt` below. Then copy `app.py` and `schema.py` to your working directory.
1) run: `FLASK_ENV=development && flask run`
2) execute query ```{
__schema {
types {
name
}
}
}``` and receive error `TypeError: UserConvo fields cannot be resolved. There is no current event loop in thread 'Thread-1'.`
3) run `FLASK_ENV=development && flask run --no-reload
4) execute query ```{
__schema {
types {
name
}
}
}``` and receive no error.

Error Trace:
```
Traceback (most recent call last):
File "/Users/erichemphill/workspace/craft/graphene-test/app.py", line 4, in
from schema import schema
File "/Users/erichemphill/workspace/craft/graphene-test/schema.py", line 74, in
schema = Schema(query=Query)
File "/Users/erichemphill/workspace/craft/graphene-test/env/lib/python3.8/site-packages/graphene/types/schema.py", line 430, in __init__
self.graphql_schema = GraphQLSchema(
File "/Users/erichemphill/workspace/craft/graphene-test/env/lib/python3.8/site-packages/graphql/type/schema.py", line 208, in __init__
collect_referenced_types(query)
File "/Users/erichemphill/workspace/craft/graphene-test/env/lib/python3.8/site-packages/graphql/type/schema.py", line 423, in collect_referenced_types
collect_referenced_types(field.type)
File "/Users/erichemphill/workspace/craft/graphene-test/env/lib/python3.8/site-packages/graphql/type/schema.py", line 422, in collect_referenced_types
for field in named_type.fields.values():
File "/opt/homebrew/Cellar/python@3.8/3.8.12_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/functools.py", line 967, in __get__
val = self.func(instance)
File "/Users/erichemphill/workspace/craft/graphene-test/env/lib/python3.8/site-packages/graphql/type/definition.py", line 737, in fields
raise TypeError(f"{self.name} fields cannot be resolved. {error}")
TypeError: UserConvo fields cannot be resolved. There is no current event loop in thread 'Thread-1'.
```

schema.py:
```
from graphene import Field, ObjectType, Schema
from graphene import relay
from graphene_sqlalchemy import SQLAlchemyObjectType
from sqlalchemy.orm import declarative_base, relationship
from sqlalchemy import Column, Integer, ForeignKey, BigInteger

Base = declarative_base()

class UserModel(Base): # pylint: disable=too-few-public-methods
"""
Models the User Object
"""

__tablename__ = "user"
user_id = Column(BigInteger, primary_key=True, autoincrement=True)
convos = relationship("UserConvoAssociation", back_populates="user")

class UserConvoAssociation(Base):
"""
Association Object for User and Convo relationships
"""

__tablename__ = "user_convo"
convo_id = Column(ForeignKey("convo.convo_id"), primary_key=True)
user_id = Column(ForeignKey("user.user_id"), primary_key=True)
num_unread_messages = Column(Integer, default=0)

convo = relationship("ConvoModel", back_populates="users")
user = relationship("UserModel", back_populates="convos")

class ConvoModel(Base): # pylint: disable=too-few-public-methods
"""
Models the Convo object
"""

__tablename__ = "convo"
convo_id = Column(BigInteger, primary_key=True, autoincrement=True)
users = relationship("UserConvoAssociation", back_populates="convo")

class UserConvo(SQLAlchemyObjectType):
"""
User x Convo relationship
"""

class Meta:
model = UserConvoAssociation
interfaces = (relay.Node,)
batching = True

class User(SQLAlchemyObjectType):
"""
Defines the schema for a User
"""

class Meta:
"""
Defines the metadata for a User
"""

model = UserModel
interfaces = (relay.Node,)
batching = True

class Query(ObjectType):
convo = Field(UserConvo)
user = Field(User)

schema = Schema(query=Query)
```

app.py

```
from flask import Flask
from graphql_server.flask import GraphQLView

from schema import schema

app = Flask(__name__)

app.add_url_rule('/graphql', view_func=GraphQLView.as_view(
'graphql',
schema=schema.graphql_schema,
graphiql=True,
))

if __name__ == '__main__':
app.run()
```

requirements.txt (this is not a minimal requirements.txt. it's from the project that i'm working on.)

```
aiodataloader==0.2.1
aiohttp==4.0.0a1
alembic==1.7.6
aniso8601==9.0.1
asgiref==3.5.0
async-timeout==3.0.1
attrs==21.4.0
aws-embedded-metrics==1.0.7
bcrypt==3.2.0
boto3==1.21.1
botocore==1.24.1
certifi==2021.10.8
cffi==1.15.0
cfn-flip==1.3.0
chardet==3.0.4
charset-normalizer==2.0.12
click==8.0.3
exponent-server-sdk==2.0.0
Flask==2.0.3
Flask-Migrate==3.1.0
Flask-SQLAlchemy==2.5.1
graphene==3.0
graphene-sqlalchemy==3.0.0b1
graphql-core==3.1.7
graphql-relay==3.1.5
graphql-server==3.0.0b4
idna==3.3
importlib-metadata==4.11.1
importlib-resources==5.4.0
itsdangerous==2.0.1
Jinja2==3.0.3
jmespath==0.10.0
Mako==1.1.6
MarkupSafe==2.0.1
multidict==4.7.6
phonenumbers==8.12.43
promise==2.3
psycopg2==2.9.3
pycparser==2.21
python-dateutil==2.8.2
python-http-client==3.3.6
PyYAML==5.3.1
requests==2.27.1
s3transfer==0.5.1
sendgrid==6.9.6
six==1.16.0
SQLAlchemy==1.4.31
starkbank-ecdsa==2.0.3
troposphere==3.2.2
typing-extensions==3.10.0.2
urllib3==1.26.8
Werkzeug==2.0.3
yarl==1.7.2
zipp==3.7.0
```