tox: reduce deps used for pep8 env

Linters are only used by the 'pep8' virtual environment, we do not need
them in other test environment.  Also 'pep8' only requires the three
linters it used and the other modules are not needed.

Move the pep8 dependencies to the pep8 testenv.

Change-Id: I3a8436cbff9e2386cf0e0f5b0d1742283e647b56

test-requirements.txt

@@ -1,11 +1,7 @@
-flake8
-openapi-spec-validator
-
 coverage>=3.6
 fixtures>=0.3.14
 stestr>=1.0.0 # Apache-2.0
 testtools>=0.9.32
 PyMySQL
 psycopg2-binary
-mypy<0.650
 beautifulsoup4

tox.ini

@@ -52,6 +52,10 @@ install_command = pip install {opts} {packages}
 # --ignore-missing-imports tells mypy to not try to follow imported modules
 # out of the current tree. As you might expect, we don't want to run static
 # type checking on the world - just on ourselves.
+deps =
+  flake8
+  mypy<0.650
+  openapi-spec-validator
 commands =
   flake8 {posargs}
   mypy --ignore-missing-imports zuul