Move CI tests from Travis to GitHub (#3889)

* Add initial support for moving tests to GitHub

* Add setup Python step in the workflow

* Remove Travis CI config file

* Fix GitHub action file for build to trigger on PR

* Use Python 3.8 as tensorflow is not yet supported

* Fix ciphers.hill_cipher doctest error

* Fix: instagram crawler tests failing on GitHub actions

* Fix floating point errors in doctest

* Small change to test cache

* Apply suggestions from code review

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update instagram_crawler.py

Co-authored-by: Christian Clauss <cclauss@me.com>