Streamline builds in preparation for Python 3.7

[Lnaden]

Start builds against Python 3.7 for testing

[jchodera]

@Lnaden : It looks like conda-forge is actively building numpy 1.9, 1.11, and 1.15 for Python 3.7 now:
https://anaconda.org/conda-forge/numpy/files

They're have also built numpy 1.10, 1.12, 1.13, and 1.14 for Python 2.7, 3.5, and 3.6.

This is a pretty weird combination.

[jchodera]

@Lnaden : I've tried to update the build infrastructure to build against the numpy versions available for each Python version on conda-forge.

[Lnaden]

Thats fine. I tried to make as minimal changes as we need to. Honestly, we could probably (and should) remove that bit since none of our packages use the numpy x.x formalism anymore. We could just remove that check. They generalized that system in Conda Build 3, but that will take re-designing that script and most the recipes. You could just build against the max version (1.15) right now.

[jchodera]

Good point!

[jchodera]

Looks like conda-forge hasn't started building some of our dependencies for Python 3.7 yet, so this might have to hold for a bit:

Can't build /io/openmm due to unsatisfiable dependencies:
The following specifications were found to be in conflict:
  - python 3.7*
  - sphinx ==1.5.6 -> python 2.7* -> openssl 1.0.1*
  - sphinx ==1.5.6 -> python 2.7* -> readline 6.2*
  - sphinx ==1.5.6 -> python 2.7* -> tk 8.5*
Use "conda info <package>" to see the dependencies for each package.

[Lnaden]

I think you can safely unpin from sphinx 1.5.6 per the PR I have here on the main conda-recipes channel. Just make sure its at least that to avoid other odd backwards compatibility issues

[jchodera]

@Lnaden: For reference, unpinning sphinx doesn't seem to work:

The LaTeX files are in ../../build/sphinx-docs/developerguide/latex.
Run 'make' in that directory to run these through (pdf)latex
(use `make latexpdf' here to do that automatically).
Running LaTeX files through pdflatex...
gmake[5]: latexmk: Command not found
gmake[5]: *** [OpenMMDeveloperGuide.pdf] Error 127
gmake[4]: *** [latexpdf] Error 2
make[3]: *** [sphinx-docs/developerguide/latex/OpenMMDeveloperGuide.pdf] Error 2
make[2]: *** [docs-source/CMakeFiles/sphinxpdf.dir/all] Error 2
make[1]: *** [docs-source/CMakeFiles/sphinxpdf.dir/rule] Error 2
make: *** [sphinxpdf] Error 2
Traceback (most recent call last):
  File "/opt/conda/bin/conda-build", line 6, in <module>
    sys.exit(conda_build.cli.main_build.main())
  File "/opt/conda/lib/python3.6/site-packages/conda_build/cli/main_build.py", line 342, in main
    execute(sys.argv[1:])
  File "/opt/conda/lib/python3.6/site-packages/conda_build/cli/main_build.py", line 333, in execute
    noverify=args.no_verify)
  File "/opt/conda/lib/python3.6/site-packages/conda_build/api.py", line 97, in build
    need_source_download=need_source_download, config=config)
  File "/opt/conda/lib/python3.6/site-packages/conda_build/build.py", line 1524, in build_tree
    config=config)
  File "/opt/conda/lib/python3.6/site-packages/conda_build/build.py", line 1147, in build
    utils.check_call_env(cmd, env=env, cwd=src_dir)
  File "/opt/conda/lib/python3.6/site-packages/conda_build/utils.py", line 628, in check_call_env
    return _func_defaulting_env_to_os_environ(subprocess.check_call, *popenargs, **kwargs)
  File "/opt/conda/lib/python3.6/site-packages/conda_build/utils.py", line 624, in _func_defaulting_env_to_os_environ
    return func(_args, **kwargs)
  File "/opt/conda/lib/python3.6/subprocess.py", line 291, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/bin/bash', '-x', '-e', '/opt/conda/conda-bld/openmm_1535835361528/work/conda_build.sh']' returned non-zero exit status 2.

[jchodera]

The problem now is that there appear to be no conda-forge sphinx builds for py37.