Downgrade pytest-twisted to 1.13

1.14 apparently caused issues with hanging tests.  Don't know why,
because pytest-twisted does not seem to publish a
proper changelog, so I have no idea what changed between 1.13 and 1.14.

Author: lunkwill42

tests/requirements.txt

@@ -9,7 +9,7 @@ pytest-metadata<2.0.0
 pytest-cov==2.7.1
 pytest-selenium==2.0.1
 pytest-timeout
-pytest-twisted~=1.14.0
+pytest-twisted~=1.13.0
 pytidylib==0.3.2
 selenium==3.141.0
 whisper>=0.9.9