Update dependencies of Python 3.5 on django 2.2 and force fixing the 2.7 builds

[NyanKiyoshi]

• A bad release of pluggy broke randomly the 2.7 builds, the version is now tight to 1.11 with this fix applied importlib_metadata breaks TravisCI builds pytest-dev/pluggy#205.
• Python 3.5 is now ran into xenial to have a more recent version of sqlite3 and of python 3.5 which was failing to build randomly as well.

[codecov-io]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           master    #2   +/-   ##
====================================
  Coverage      50%   50%           
====================================
  Files           4     4           
  Lines          36    36           
====================================
  Hits           18    18           
  Misses         18    18

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