[Tox] Pin pip version to fix PEP517 issues

 * Using pyproject.toml for black config pip version 19.1 errors out
   about using editable install with pyproject.toml.
   Workaround is to not use pip 19.1 in tox.
 * Pin to 18.1 to avoid pip-wheel-metadata-folder creation

Ref:
 - pypa/pip#6434
 - pypa/pip#6213

tox.ini

@@ -18,8 +18,8 @@ addopts = -p no:warnings --basetemp=_pytest_temp
 [basesetup]
 # Minimum pip and setuptools versions to fix system and travis issues.
 deps =
-    pip>=10
-    setuptools>=40
+    pip >= 10, <= 18.1
+    setuptools >= 40
 
 [basetests]
 deps =