LGTM: Start testing on Python 3 instead of Python 2 (#510)

* LGTM: Start testing on Python 3 instead of Python 2

Fixes: #492

* LGTM: Run flake8 tests in before_index

* Update .lgtm.yml

* Indentation

* python3 -m

* Try on Python 3.7 instead of 3.6

* LGTM: Run flake8 tests on Python 3.6

* Check version of flake8

* python3 -m pip install --upgrade --user flake8

* requirements: flake8

* Update .lgtm.yml

* Ready for review

* Update .lgtm.yml

* Force a retest

* Rebuild