Fix python version detection on Debian testing/unstable
Fix python version detection on Debian testing/unstable

The current 2.7 Python binary on Debian returns "2.7.15+" as its version, which
is not PEP440 compliant. Tthere is no feedback at all from Debian maintainers
regarding the bug report as of now:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914072

This patch makes sure to drop any trailing '+' character in a version string.

Fixes #3049

foosel/OctoPrint,Jaesin/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint