RowProxy.__getitem__() should accept ColumnElement objects, not just Column

`RowProxy.__getitem__()`, works [by looking up elements either on its own `_keymap` or on `_parent.keys`](https://github.com/zzzeek/sqlalchemy/blob/2734439fff953a7bb8aecdedb5f851441b5122e9/lib/sqlalchemy/engine/result.py#L94-L114). In the case of a `select` statement, the resulting RowProxy object will have keys matching the `ColumnElement`s that were passed to that select, not the initial `Column` objects, so the signature for `__getitem__` should read:

```py
def __getitem__(self, key: Union[str, int, ColumnElement]) -> Any: ...
```

...instead of:

https://github.com/dropbox/sqlalchemy-stubs/blob/1c36a4ef0898c9bbd0824c0cd2ae8e6b8542930f/sqlalchemy-stubs/engine/result.pyi#L19

----

Working example:

```py
from sqlalchemy import create_engine, MetaData, Table, Column, select, INTEGER, TEXT

engine = create_engine("sqlite://", echo=True)
test_metadata = MetaData()

test_column = Column('test_id', INTEGER, primary_key=True)
test_table = Table('test_table', test_metadata, test_column)

test_metadata.create_all(engine)

engine.execute(test_table.insert(), [{'test_id': num} for num in range(10)])

res1 = engine.execute(select([test_column]))

for row in res1:
print("BY LABEL")
print('\t', row['test_id'])
print("BY COLUMN")
print('\t', row[test_column])

test_cast = test_column.cast(TEXT) # Cast[int]/ColumnElement

res2 = engine.execute(select([test_cast]))

for row in res2:
# The returned label is 'anon_1', not 'test_id'
# row[test_column] does not exist
print("BY COLUMN ELEMENT")
print('\t', row[test_cast]) # Invalid index type "Cast[int]" for "RowProxy"
```