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

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

Fixes: TheAlgorithms#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

.lgtm.yml

@@ -0,0 +1,12 @@
+extraction:
+  python:
+    python_setup:
+      version: 3
+    after_prepare:
+      - python3 -m pip install --upgrade --user flake8
+    before_index:
+      - python3 -m flake8 --version  # flake8 3.6.0 on CPython 3.6.5 on Linux
+      # stop the build if there are Python syntax errors or undefined names
+      - python3 -m flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+      # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+      - python3 -m flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics