WIP, TST: remove gsd & add Python 2.7 to appveyor config

[tylerjereddy]

IIRC the need to "mock import" gsd was preventing us from building & testing MDAnalysis with Python 2.7 on Windows (gsd could be built but not actually used in 3.6, but not even built in 2.7 I think).

This follows up on #2090 to see if Appveyor is happy with Python 2.7 builds now.

[tylerjereddy]

I guess that was a tad optimistic

[codecov]

Codecov Report

Merging #2093 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2093   +/-   ##
========================================
  Coverage    89.41%   89.41%           
========================================
  Files          159      159           
  Lines        18737    18737           
  Branches      2698     2698           
========================================
  Hits         16753    16753           
  Misses        1381     1381           
  Partials       603      603

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[tylerjereddy]

what motivated this change? if we want to support Python 2.7 on Windows we need to use older compilers and enforce C89 standard in our C source files I think. Maybe more trouble than it is worth though.

[richardjgowers]

I was just playing spot the difference between the two builds and hoping it was a typo. I didn't realise we had fundamental problems like that.

[kain88-de]

Did we decide to drop 2.7 for windows? We are not going for the windows build with 2.7 on conda-forge

[richardjgowers]

@kain88-de it won't work atm as far as I know, if someone did a PR that fixed it then sure, so it's more a known issue than we've dropped 2.7 support on Windows

[tylerjereddy]

We'd need to revert to C89 standard to get compilation working here so closing based on previous discussions.