Pin all the test requirements to try and make testing deterministic

requirements-test.txt

@@ -1,10 +1,39 @@
 -r requirements.txt
 
-tox
+# make sure we have a deterministic test setup
 
+# pin all of our direct dependencies
+tox==3.14.5
 flake8==3.7.9
+pytest==3.2.5,<4 # see https://github.com/html5lib/html5lib-python/pull/429
+coverage==5.0.3
+pytest-expect==1.1.0
+mock==3.0.5
 
-pytest==3.2.5
-coverage
-pytest-expect>=1.1,<2.0
-mock
+# and all recursive dependencies
+appdirs==1.4.3
+configparser==4.0.2
+contextlib2==0.6.0.post1
+distlib==0.3.0
+entrypoints==0.3
+enum34==1.1.9
+filelock==3.0.12
+funcsigs==1.0.2
+functools32==3.2.3.post2 ; python_version < '3'
+importlib-metadata==1.5.0
+importlib-resources==1.0.2
+mccabe==0.6.1
+packaging==20.1
+pathlib2==2.3.5
+pluggy==0.13.1
+py==1.8.1
+pycodestyle==2.5.0
+pyflakes==2.1.1
+pyparsing==2.4.6
+scandir==1.10.0
+# six==1.14.0 # ignored because it's also in requirements.txt
+toml==0.10.0
+typing==3.7.4.1
+u-msgpack-python==2.5.2
+virtualenv==20.0.6
+zipp==1.2.0