mypy: attempt to fix build

Author: hauntsaninja

.travis.yml

@@ -81,6 +81,7 @@ jobs:
 
 install:
 - pip install -U pip setuptools
+- pip install -U 'virtualenv<20'
 - pip install -U tox==3.9.0
 - tox --notest
 

appveyor.yml

@@ -14,7 +14,7 @@ environment:
 
 install:
     - "git submodule update --init mypy/typeshed"
-    - "%PYTHON%\\python.exe -m pip install -U setuptools tox==3.9.0"
+    - "%PYTHON%\\python.exe -m pip install -U setuptools 'virtualenv<20' tox==3.9.0"
     - "%PYTHON%\\python.exe -m tox -e py37 --notest"
 
 build: off