fix: update flake8 and deps to work with Python 3.8+ (elastic#1085)

This fixes 'make lint' when one has Python 3.8 or later
(PyCQA/pyflakes#367), but updating to the
latest flake8 and its deps. Note that flake8 3.9.0 dropped support
for Python 3.4
(https://flake8.pycqa.org/en/latest/release-notes/3.9.0.html)

requirements.txt

@@ -11,16 +11,16 @@ docker==4.3.1
 dockerpty==0.4.1
 docopt==0.6.2
 elasticsearch==6.3.1
-flake8==3.6.0
+flake8==3.9.0
 funcsigs==1.0.2
 future==0.16.0
 idna==2.6
 jsonschema==2.6.0
 mccabe==0.6.1
 pluggy==0.13.1
 py==1.5.2
-pycodestyle==2.4.0
-pyflakes==2.0.0
+pycodestyle==2.7.0
+pyflakes==2.3.0
 pytest-base-url==1.4.2
 pytest-html==2.1.1
 pytest-metadata==1.10.0