Retool for invocation from MINGW{32,64}, not MSYS

[Andrew Chadwick]

Styrene needs to think about its package-level dependencies. In MSYS2, packages from the mingw32 & mingw64 repositories may depend on packages from the msys2 repository. The reverse is not true.

I'd still like to use the msys2 Python interpreter, at least until we have all of styrene's dependencies in MINGWxx python packages. Converting to non-Cygwinlike native python is more work than this issue proposes, but could be done at a later date with care.

Quite a bit of the styrene internals will need to be fixed:

1. Check for invocation from os.environ["MSYSTEM"].startswith("MINGW")
2. Unify "native subshell" and regular subprocess invocations.
3. This will require changes in #4 to be rolled back. Thinking about a central wrapper script, though.
4. Need to rejigger the TeaCI matrix since everything will have changed.

[Andrew Chadwick]

That'll do for starters.