Upgrade to Python 3.6.8

[slel]

Python 3.6.8 was released on 2018-12-24: Python insider blog post: Python 3.6.8.

Tarball: https://www.python.org/ftp/python/3.6.8/Python-3.6.8.tar.xz

Notes:

• our last upgrade was to Python 3.6.6 in Upgrade to Python 3.6.6 #25771
• upgrading to Python 3.7.x is tracked at Upgrade to Python 3.7.x #25680

CC: @antonio-rojas @fchapoton @dimpase @embray @kiwifb @jdemeyer @jhpalmieri @mkoeppe @saraedum @slel @tscrim @vbraun @vinklein

Component: packages: standard

Keywords: upgrade

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/26538

[vinklein]

comment:1

Attachment: testalllong_with_py.3.6.7.log

Add an attachment. sage -t -a --long logs on ubuntu, with a sage compiled with python3.6.7.

I haven't made the comparison with a python3.6.6 log yet.

[slel]

comment:2

Python 3.6.8rc1 was released: Python insider blog post: Python 3.6.8rc1.

[slel]

comment:3

Python 3.6.8 is now available.

[embray]

comment:4

Ticket retargeted after milestone closed (if you don't believe this ticket is appropriate for the Sage 8.8 release please retarget manually)

[fchapoton]

comment:5

update to 3.7.3 done in #25680

[tscrim]

Reviewer: Travis Scrimshaw