Can we deprecate portable_popen and locate_executable?

[Jeff Dairiki]

Lektor currently provides lektor.utils.portable_popen and lektor.utils.locate_executable.

Locate_executable seems to be largely identical or equivalent to the standard library shutil.which.

Does subprocess.Popen really still need help to work on Windows?
(The big red warning on the Popen docs does say, essentially, that any path modifications implied by the cwd or env parameters to Popen may not work as desired on Windows — the advice being to resolve the full path to the executable manually using shutil.which. I'm not sure it's an issue at all if the cwd or env parameters are not being used.)