Pin `protobuf<4.21.0`

[thisac]

New version of protobuf is released causing the documentation to fail.

/opt/hostedtoolcache/Python/3.7.13/x64/lib/python3.7/site-packages/google/protobuf/descriptor.py:560: in __new__
    _message.Message._CheckCalledFromGeneratedFile()
E   TypeError: Descriptors cannot not be created directly.
E   If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
E   If you cannot immediately regenerate your protos, some other possible workarounds are:
E    1. Downgrade the protobuf package to 3.20.x or lower.
E    2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).

The temporary hotfix is to pin protobuf<4.21.0. However, this should be considered temporary, and be undone if/when protobuf --- or more likely, our primary dependencies such as TensorFlow --- releases a bugfix.

[codecov[bot]]

Codecov Report

Merging #707 (f6245bb) into master (51af6ce) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #707   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          75       75           
  Lines        9225     9225           
=======================================
  Hits         9107     9107           
  Misses        118      118           

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