Update Python and dev dependency versions

Author: FichteFoll

tox.ini

@@ -1,10 +1,10 @@
 [tox]
-envlist = py34,py35,py36,py37,flake8
+envlist = py35,py36,py37,py38,flake8
 
 [testenv]
 deps =
-  pytest==4.1.1
-  coverage==4.5.2
+  pytest==5.3.5
+  coverage==5.0.3
 commands =
   coverage run -m pytest {posargs}
   # coverage report -m
@@ -18,7 +18,7 @@ testpaths =
 skipsdist = True
 basepython = python3
 deps =
-  flake8==3.7.7
+  flake8==3.7.9
   # flake8-docstrings==1.3.0
 commands =
   flake8 {posargs} .