Dev dependencies now require old pip version that tox can work with.

setup.py

@@ -51,6 +51,7 @@ setup(
         # even though I could install a lot of viruses just from this setup.py
         # "linux": ["PyV4L2Cam @ git+https://github.com/SimLeek/PyV4L2Cam"],
         "dev": [
+            "pip==18.1"
             "black==18.*,>=18.3.0.a0",
             "coverage==4.*,>=4.5.0",
             "mock==3.*,>=3.0.0",

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36, py37, pydocstyle
+envlist = py36, py37, mypy, pydocstyle
 isolated_build = false
 skip_missing_interpreters = true
 skipsdist=True
@@ -7,7 +7,7 @@ skipsdist=True
 [gh-actions]
 python =
     3.6: py36, pydocstyle
-    3.7: py37, pydocstyle
+    3.7: py37, mypy, pydocstyle
 
 [testenv]
 whitelist_externals = coverage