main: require Python 3.5 now

We've been warning about Python 3.4 for almost a year.  This drops
support for these systems:
* Ubuntu Trusty: released Apr 2014, EOL Apr 2022
* Debian Jessie: released Apr 2015, EOL Jun 2020

So the min required distros would now be:
* Ubuntu Xenial: released Sep 2015 w/Python 3.5
* Debian Stretch: released Jun 2017 w/Python 3.6

I don't think we're quite ready to drop Python 3.5 which would affect
Ubuntu Xenial -- we'd have to update to Ubuntu Bionic from Apr 2018.
Let's see how much the community reacts to loss of Python 3.4 first.

Bug: https://crbug.com/gerrit/10418
Change-Id: Ib24a57818fdca49e23db53e1bdd1f4c76b4963f7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/291502
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

main.py

@@ -83,7 +83,7 @@ if not is_python3():
 #
 # python-3.6 is in Ubuntu Bionic.
 MIN_PYTHON_VERSION_SOFT = (3, 6)
-MIN_PYTHON_VERSION_HARD = (3, 4)
+MIN_PYTHON_VERSION_HARD = (3, 5)
 
 if sys.version_info.major < 3:
   print('repo: error: Python 2 is no longer supported; '