Unify querying of executable versions

[anntzer]

PR Summary

Provide get_executable_info to replace the various checkdep_foo checkers (now deprecated), as suggested in #9571. Provide get_all_executable_infos for the purpose of quickly checking installed versions of all "relevant" executables.

Bump minimal supported ghostscript version to 9.0 (released in 2010, https://www.ghostscript.com/doc/current/History9.htm) in order to remove a conditional path.

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

Is the minimal gs version in the docs anyplace that also needs to be update?

[anntzer]

Not as far as I can see.

[efiring]

It looks like this needs a little bit of updating to remove Py2.7 compatibility, but overall it looks good.

[anntzer]

py2.7 issues are handled. (I'm leaving the import reordering as is to keep the PR focused(ish).)

[timhoffm]

That's just a caching mechanism, which is obsolete because get_executable_info() ls lru_cached.

[anntzer]

I know, but it doesn't cost me anything to keep it around until checkdep_inkscape itself gets removed.

[timhoffm]

Also None if the version smaller than the minimal required version. Which can crash later usage patterns like str(get_executable_info("dvipng").version).

I think, this function does too much. The version check should not be part of getting the version.

[anntzer]

Good points. I'll try to come up with a better API.

[anntzer]

What about

1. require_executable(exec_name) which returns the correct name to invoke the executable if it exists and its version is recent enough, and throws some exception with a reasonable error message if either the executable doesn't exist, or its version is too old;
2. list_executables() which lists install status and version info for all "relevant" executables (mostly to get debugging info; I actually don't particularly think that's needed but I promised that to get Remove LaTeX checking in setup.py. #9571 in :-) or we could even just have a mpl.get_debug_info() similar to pandas.show_versions() which just dumps all kinds of whatever we think may be relevant in bug reports: versions of dependencies, versions of IPython/Jupyter, version of external executables, etc.)

Note that while 1) may seem a bit overkill just for ghostscript, there's actually at least another tool that would benefit from this, which is convert (imagemagick) in the animation framework), which currently also involves a sophisticated dance on Windows.

[anntzer]

Superseded by #13303.