Relax Python version but pin dependencies in tox linter environments

Author: althonos

tox.ini

@@ -17,24 +17,22 @@ deps =
   !scandir: .
 
 [testenv:mypy]
-basepython = python3.9
 commands = mypy --config-file {toxinidir}/setup.cfg {toxinidir}/fs
 deps =
   .
   mypy==0.800
 
 [testenv:flake8]
-python = python3.9
 commands = flake8 {toxinidir}/fs {toxinidir}/tests
 deps =
-  flake8
-  flake8-bugbear
-  flake8-builtins
-  flake8-comprehensions
-  flake8-mutable
+  flake8==3.7.9
+  #flake8-builtins==1.5.3
+  flake8-bugbear==19.8.0
+  flake8-comprehensions==3.1.4
+  flake8-mutable==1.2.0
+  flake8-tuple==0.4.0
 
 [testenv:black]
-python = python3.9
 commands = black --check {toxinidir}/fs
 deps =
   black==20.8b1