Update .appveyor.yml

[justoruiz]

Make sure you have checked all steps below.

Description

Upgrade appveyor so it uses Visual Studio 16 2019 and fixes issues with python; it also builds khiva from scratch.

Tests

See the new script successfully run all tests.

Commits

A series of commits in this branch until the script run successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (08b7602) to head (a02d831).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #36   +/-   ##
=========================================
  Coverage     92.37%   92.37%           
  Complexity      219      219           
=========================================
  Files            17       17           
  Lines           472      472           
  Branches         18       18           
=========================================
  Hits            436      436           
  Misses           22       22           
  Partials         14       14           

Flag	Coverage Δ
Linux	92.37% <ø> (ø)
MacOS	?
Windows	?

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