Use Tox to specify the [watchmedo] extra

This resolves an error that occurs in CI with new Python software,
which doesn't understand the `-e .[watchmedo]` syntax
in `requirements-tests.txt`.

Author: kurtmckee

requirements-tests.txt

@@ -1,4 +1,3 @@
--e ".[watchmedo]"
 eventlet
 flake8
 flaky

tox.ini

@@ -6,19 +6,25 @@ skip_missing_interpreters = True
 usedevelop = true
 deps =
     -r requirements-tests.txt
+extras =
+    watchmedo
 commands =
     python -bb -m pytest {posargs}
 
 [testenv:flake8]
 usedevelop = true
 deps =
     -r requirements-tests.txt
+extras =
+    watchmedo
 commands =
     python -m flake8 docs tools src tests setup.py
 
 [testenv:docs]
 usedevelop = true
 deps =
     -r requirements-tests.txt
+extras =
+    watchmedo
 commands =
     sphinx-build -aEWb html docs/source docs/build/html