CI: add more github actions

[benbovy]

TODO:

• remove travis completely
• update badges in README (tests and coverage)
• wait for codecov to support github actions without token (sending coverage from PRs is currently not possible due to this limitation)

See https://community.codecov.io/t/whitelist-github-action-servers-to-upload-without-a-token/491/16

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3138f35). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #86   +/-   ##
=========================================
  Coverage          ?   99.33%           
=========================================
  Files             ?       11           
  Lines             ?     1498           
  Branches          ?        0           
=========================================
  Hits              ?     1488           
  Misses            ?       10           
  Partials          ?        0

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