main: require Python 3.5 now
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 <4ca9f429749ea29f0277a76979656ec13beb5513@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

GerritCodeReview/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,lipro-yocto/git-repo,couchbasedeps/git-repo,lipro-yocto/git-repo