Fix pyenv FAQ env var commands

[cas--]

When using pyenv and global not set (defaulting to system) or global set to system the Launcher was unable to determine correct version to use.

Fixed by using pyenv exec to correctly determine current Python version instead of parsing pyenv config files.

Fixes #141

[brettcannon]

One tricky thing with this compared to the old solution is PY_PYTHON is the default version to use when no specific major version is requested, e.g. it's not the same as PY_PYTHON2. That means this isn't technically right as you have to make an assumption as to what major version of Python people want to depend on compared to the solution of reading (pyenv root)/version).

Maybe it's better to list both solutions?

[cas--]

I merged your suggestions but then changed my mind and reverted as I think there has been a misunderstanding.

One tricky thing with this compared to the old solution is PY_PYTHON is the default version to use when no specific major version is requested e.g. it's not the same as PY_PYTHON2

The use of python is not meant to refer to python2. Using $(pyenv exec python) solution mirrors the old solution since pyenv python is what the user set via pyenv global (the (pyenv root)/version file) or fallback to system.

I hope this make sense?

[brettcannon]

Then please drop the PY_PYTHON3 example as I don't think many people will care enough to warrant making the example more complex.

[cas--]

I agree, was including it for completeness for -3 option

[brettcannon]

I just realized the changes were made against the main copy of the README and not the template which the README is generated from. Would you mind applying the changes to https://github.com/brettcannon/python-launcher/blob/main/docs/readme/template.md instead?

[brettcannon]

Thanks for the help with this!