Replace --python with --try-first-with (#9413)

If a tool like asdf provides a python that is first in the path, virtualenv does not use it even though it is the one
used to invoke poetry.  Using --try-first-with (introduced in 20.3.0) allows poetry to point to the prefered python.

See https://virtualenv.pypa.io/en/latest/changelog.html#v20-3-0-2021-01-10
fixes python-poetry#9278