[TST] GitHub Actions only; Migrate away from Travis/Appveyor

[jGaboardi]

This PR embodies a large break in the current standard PySAL testing schema.

Migration Highlights

• significantly less complex
• tests ubuntu, mac, windows
• tests Python 3.6, 3.7, 3.8
• tests both hard and optional requirements
• tests with both the current libpysal release and the current master
• performs code-cov
• run nightly tests
• negates dual dependency on Travis and Appveyor for testing

[codecov]

Codecov Report

Merging #453 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #453   +/-   ##
=======================================
  Coverage   97.85%   97.85%           
=======================================
  Files           3        3           
  Lines        1120     1120           
  Branches      269      269           
=======================================
  Hits         1096     1096           
  Misses          6        6           
  Partials       18       18

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[jGaboardi]

@sjsrey I'm not sure why random builds in the matrix hang/stall, but I have a question about in to the GitHub Community Forum. The stalling builds are generally different each time, with some workflow runs passing completely without stalled builds. This PR successfully disabled Travis, but Appveyor is still being triggered (even though I disabled the webhook). I think we should merge this see how it goes.

[jGaboardi]

All GitHub actions pass after rerunning several times. Merging. Will try to debug stalls/hangups later.