Fix versions in flake8 check-manifest commands

tox.ini

@@ -116,7 +116,7 @@ skip_install=True
 deps =
     check-manifest==0.41
 commands =
-    check-manifest==0.41
+    check-manifest
 
 [testenv:check_codestyle]
 skip_install = True
@@ -128,7 +128,7 @@ deps =
     black==19.10b0  # We pin so that our tests don't start failing on new releases of black.
 commands =
     python -m black --check --diff .
-    /bin/sh -c "flake8==3.7.9 synapse tests scripts scripts-dev synctl {env:PEP8SUFFIX:}"
+    /bin/sh -c "flake8 synapse tests scripts scripts-dev synctl {env:PEP8SUFFIX:}"
     {toxinidir}/scripts-dev/config-lint.sh
 
 [testenv:check_isort]