chore(main): release 1.20.0

[release-please]

🤖 I have created a release beep boop

1.20.0 (2022-02-07)

Features

• add custom dir for messages and message classes (#289) (35e019e)

Bug Fixes

• workaround for buggy pytest (#291) (28aa3b2)

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@@           Coverage Diff            @@
##             main      #292   +/-   ##
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  Coverage        ?   100.00%           
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  Files           ?        22           
  Lines           ?       968           
  Branches        ?       214           
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  Hits            ?       968           
  Misses          ?         0           
  Partials        ?         0           

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[yoshi-automation]

Release is at https://github.com/googleapis/proto-plus-python/releases/tag/v1.20.0