docs: link to pypi protobuf package

[busunkim96]

Release-As: 1.10.0

Creating a release for the stringy enum change previously released in 1.10.0.dev2. #118

The tests for the following libraries are green when run with the current dev release. (Most of these libraries were chosen because they previously had failing tests).

• test: use dev release of proto plus (do not merge) python-firestore#193
• test: use dev release of proto plus (do not merge) python-automl#77
• test: use dev release of proto plus python-asset#90
• test: use dev release of proto plus python-dlp#56

The Spanner folks also reported the dev release resolved the stringy enums issue without creating new test failures.

[codecov]

Codecov Report

Merging #131 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #131   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          857       857           
  Branches       149       149           
=========================================
  Hits           857       857           

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