Remove appveyor CI in favor of travis Windows and add Python 3.9 wheels

[djhoese]

Appveyor support is deprecated by ci-helpers which we are using to easily download and install conda packages. We already have Windows being tested on travis but it is an allowed failure. This PR removes appveyor and moves the Windows environment out of the allowed failure on Travis.

[codecov]

Codecov Report

Merging #310 (4b175b9) into master (8b45836) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #310   +/-   ##
=======================================
  Coverage   92.66%   92.66%           
=======================================
  Files          45       45           
  Lines        9146     9146           
=======================================
  Hits         8475     8475           
  Misses        671      671           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8b45836...4b175b9. Read the comment docs.

[coveralls]

Coverage remained the same at 92.642% when pulling e13cebb on djhoese:ci-remove-appveyor into 8b45836 on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 92.663% when pulling 4b175b9 on djhoese:ci-remove-appveyor into 8b45836 on pytroll:master.

[djhoese]

One downside I'm realizing after all of these changes is that Python 3.9 wheels still aren't built because we are waiting on a pykdtree wheel (installing from source currently fails on the wheel docker images).

[djhoese]

This is ready for review otherwise.

[pnuu]

LGTM