Fix: Update pyenv version selection

lf-pyver() fails to include the currently selected version in the output
of 'pyenv versions', which makes the version change every time the local
version is set by pyenv with the version from lf-pyver().

Fix the command to extract the list of Python versions to include all
the numeric versions in the list.

Issue-ID: RELENG-4403
Change-Id: I8eb1a1842a9ccd7514f096ec6989559a5526c3b6
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>