requirements: update 'flake8'

(plus some dependencies)

Changelog:
- 3.7.6 (2019-02-18)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.6.rst
- 3.7.5 (2019-02-04)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.5.rst
- 3.7.4 (2019-01-31)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.4.rst
- 3.7.3 (2019-01-30)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.3.rst
- 3.7.2 (2019-01-30)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.2.rst
- 3.7.1 (2019-01-30)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.1.rst
- 3.7.0 (2019-01-29)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.7.0.rst
- 3.6.0 (2018-10-23)
  https://gitlab.com/pycqa/flake8/blob/master/docs/source/release-notes/3.6.0.rst

Code diff:
https://gitlab.com/pycqa/flake8/compare/3.5.0...3.7.6

Author: glarrain

requirements/test.txt

@@ -3,7 +3,7 @@
 
 # Required packages:
 coverage==4.5.2
-flake8==3.5.0
+flake8==3.7.6
 mypy==0.670
 #requests-mock==1.5.2
 #tox==3.2.1
@@ -24,7 +24,7 @@ mccabe==0.6.1
 mypy-extensions==0.4.1
 #pluggy==0.7.1
 #py==1.5.4
-pycodestyle==2.3.1
-pyflakes==1.6.0
+pycodestyle==2.5.0
+pyflakes==2.1.0
 typed-ast==1.3.1
 #virtualenv==16.0.0