chore: release 1.19.4

[release-please]

🤖 I have created a release *beep* *boop*

1.19.4 (2021-10-08)

Documentation

• clarify that proto plus messages are not pickleable (#260) (6e691dc)

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This PR was generated with Release Please. See documentation.

[codecov]

Codecov Report

Merging #262 (b9948b9) into master (6a43682) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #262   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        21    +1     
  Lines          862       933   +71     
  Branches       149       169   +20     
=========================================
+ Hits           862       933   +71     

Impacted Files	Coverage Δ
proto/modules.py	100.00% <ø> (ø)
proto/enums.py	100.00% <100.00%> (ø)
proto/fields.py	100.00% <100.00%> (ø)
proto/marshal/collections/repeated.py	100.00% <100.00%> (ø)
proto/marshal/marshal.py	100.00% <100.00%> (ø)
proto/marshal/rules/bytes.py	100.00% <100.00%> (ø)
proto/message.py	100.00% <100.00%> (ø)

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[yoshi-automation]

Release is at https://github.com/googleapis/proto-plus-python/releases/tag/v1.19.4