Add Windows builds to Azure

[Anthchirp]

this doesn't produce working Windows builds yet, but lays some
groundwork. I had previously prepared these changes in a branch,
but they might as well move over into the main repository.

With 3.2 branched out of the way now seems an good time to touch the bootstrap script.

[codecov]

Codecov Report

Merging #1469 into master will decrease coverage by 0.20%.
The diff coverage is 20.68%.

@@            Coverage Diff             @@
##           master    #1469      +/-   ##
==========================================
- Coverage   65.81%   65.61%   -0.21%     
==========================================
  Files         614      614              
  Lines       69410    69817     +407     
  Branches     9508     9674     +166     
==========================================
+ Hits        45685    45811     +126     
- Misses      21887    22159     +272     
- Partials     1838     1847       +9     

[Anthchirp]

So: this works.

Few caveats:

• This provides only 'light' testing on Windows
  • No regression data (for reasons that will become clear shortly)
  • 7 tests marked as xfail which need investigating
  • This only adds a single Windows build job, running on Python 3.6.
• The environmental overhead is considerable
  • It takes 5 minutes to downgrade the pre-installed Visual Studio compilers to an older version because cbflib_adaptbx doesn't with with current generation compilers. Possible that this can be fixed, but I'm no expert on this. The plan is to get rid of cbflib_adaptbx anyway, which would probably make this go away.
  • It then takes 5 minutes to set up the Python environment. In the Linux/MacOS builds we can cache this step. Here the caching works fine, too - but when the environment is loaded from the cache it causes the cache process to crash. We can sort this, but not now.
  • While the build itself is cached, that is a very slow step. The reason for that is that the build directory is in excess of 4 GB. It is not clear why the build directory is so massive. We are likely doing something very wrong in our build. On Azure/Linux the build directory weighs in at 500 MB.
  • As the overall space limitation still applies, we should ideally address the size issue before even looking at regression testing.
  • Surprise: there are no Windows scipy builds on conda-forge. scikit-learn is also missing as it is downstream. So we pip install it instead. There is some work ongoing on this at conda-forge, but don't expect miracles any time soon. (scipy on Windows is hitting maximum build runtime limits on their CI)
• Other notes:
  • make reconf does not work on Windows?!
  • I'm not even convinced that make does anything?! This needs verifying.
  • While we do generate code coverage statistics we don't upload them. codecov-exe should be useful here.
• All in all this is still a very useful first step to some proper Windows build coverage.