Added missing dependency to pyproject.toml

[danieledwardgeorgehitchcock]

When running mypy against the generated client in a fresh environment, the following warnings are raised:

graphql_client\async_base_client.py:19: error: Cannot find implementation or library stub for module named "websockets.client"  [import]
graphql_client\async_base_client.py:19: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
graphql_client\async_base_client.py:20: error: Cannot find implementation or library stub for module named "websockets.typing"  [import]

The GitHub test flow is set to ignore import errors due to false positives in other libraries however, this is explicitly required in async_base_client.py

I have added the missing dependency to the pyproject.toml, have tested locally and this error then disappears when included.

[mat-sop]

We have websockets~=11.0 as an optional subscriptions dependency. We should modify async_base_client.py to pass mypy --strict even without installed websockets.

[rafalp]

Superseded by #246