Pin tox version to 3.7.0 (python#6597)

msullivan · JukkaL · commit f6d206ff5817 · 2019-03-29T11:57:49.000Z
tox 3.8.0 broke something about how we specified dependencies.
I've filed a bug with tox, but let's just pin versions for now.
diff --git a/.travis.yml b/.travis.yml
@@ -51,7 +51,7 @@ jobs:
 
 install:
 - pip install -U pip setuptools
-- pip install -U tox
+- pip install -U tox==3.7.0
 - tox --notest
 
 script:
diff --git a/appveyor.yml b/appveyor.yml
@@ -10,7 +10,7 @@ environment:
 
 install:
     - "git submodule update --init mypy/typeshed"
-    - "%PYTHON%\\python.exe -m pip install -U setuptools tox"
+    - "%PYTHON%\\python.exe -m pip install -U setuptools tox==3.7.0"
     - "%PYTHON%\\python.exe -m tox -e py37 --notest"
 
 build: off
