use pip-compile to pin dev requirements

tox.ini

@@ -8,10 +8,7 @@ skip_missing_interpreters = true
 
 [testenv]
 deps =
-    pytest
-    greenlet
-    blinker
-    python-dotenv
+    -r requirements/tests.txt
 
     lowest: Werkzeug==0.15.5
     lowest: Jinja2==2.10
@@ -36,5 +33,5 @@ skip_install = true
 commands = pre-commit run --all-files --show-diff-on-failure
 
 [testenv:docs]
-deps = -r docs/requirements.txt
+deps = -r requirements/docs.txt
 commands = sphinx-build -W -b html -d {envtmpdir}/doctrees docs {envtmpdir}/html