Import problem

* Operating System version: docker image python:3.9 (debian bullseye)
* Firebase SDK version: firebase_admin-5.0.3
* Firebase Product: auth
* Python version: 3.9
* Pip version: 21.2.4

#### Steps to reproduce:
Hello everyone!
I've got some import problem while importing auth module today:
```ImportError while loading conftest '/builds/haut.ai-team/A5ns352s/1/haut.ai-team/sasuke/backend/saas_core/source/tests/conftest.py'.
source/tests/conftest.py:7: in
import firebase_admin.auth
/usr/local/lib/python3.9/site-packages/firebase_admin/auth.py:22: in
from firebase_admin import _auth_client
/usr/local/lib/python3.9/site-packages/firebase_admin/_auth_client.py:20: in
from firebase_admin import _auth_providers
/usr/local/lib/python3.9/site-packages/firebase_admin/_auth_providers.py:21: in
from firebase_admin import _auth_utils
/usr/local/lib/python3.9/site-packages/firebase_admin/_auth_utils.py:23: in
from firebase_admin import _utils
/usr/local/lib/python3.9/site-packages/firebase_admin/_utils.py:23: in
import httplib2
/usr/local/lib/python3.9/site-packages/httplib2/__init__.py:52: in
from . import auth
/usr/local/lib/python3.9/site-packages/httplib2/auth.py:20: in
auth_param_name = token.copy().setName("auth-param-name").addParseAction(pp.downcaseTokens)
E AttributeError: module 'pyparsing' has no attribute 'downcaseTokens'
```

#### Relevant Code:

```
import firebase_admin.auth
```