PYTHON environment variables are ignored

Python defines a number of environment variables which we currently ignore. This is a placeholder issue for any tests failing because of this.

My initial thought is that ignoring the environment variables is probably fine when embedding in .NET but that we should probably handle the ones that make sense when running ipy.

We currently handle some of the variables with a prefix "IRON" (e.g. IRONPYTHONPATH) where it makes sense, but it's probably not necessary to have a prefix all the time (e.g. PYTHONVERBOSE).

Specific examples which I've run into recently:

• PYTHONVERBOSE: this is ignored even though it's listed in the cli help (I'll file a PR to remove it from the help).
• PYTHONUTF8: I'm currently adding utf8 mode (new in 3.7) which can be enabled via a cli argument or an option, however I will probably not add support for the environment variable in the initial implementation.
• PYTHONIOENCODING: not implemented...

Start with the Python environment-variable documentation linked in the issue and review the existing IRON-prefixed variable handling. Define which variables should apply when running ipy rather than embedding in .NET, then verify that supported variables and CLI help are consistent.