tests: fix pip backtracking

* With the current setup of minimizing the number of different
  virtualenvs used by tox we ended up hitting an issue of pip
  backtracking. As prospector seems to be the most likely culprit here
  because has a lot of dependencies, and in the past too we had issues
  between prospector and flake8 dependencies, move prospector to its own
  virtualenv.
* Add also mypy as an explicit dependency.

Based off of I0bcbb448ef41 by Volans.

Change-Id: I024dcf05118b3c531edaec12ffd9ca570994f83a

setup.py

@@ -31,7 +31,7 @@
         'bandit>=1.5.1',
         'flake8>=3.6.0',
         'flake8-import-order>=0.18.1',
-        'prospector[with_everything]>=1.3.1',
+        'mypy',
         'pytest-cov>=2.6.0',
         'pytest-xdist>=1.26.1',
         'pytest>=3.10.1',
@@ -40,12 +40,18 @@
         'sphinx-argparse>=0.2.2',
         'Sphinx>=1.8.4',
     ],
+    'prospector': [
+        'prospector[with_everything]>=1.3.1',
+        'pytest>=3.10.1',
+        'requests-mock>=1.5.2',
+    ],
 }
 
 # Copy tests requirements to test only base dependencies
 extras_require['tests-base'] = extras_require['tests'][:]
 # Add optional dependencies to the tests ones
 extras_require['tests'].extend(extras_require['with-openstack'])
+extras_require['prospector'].extend(extras_require['with-openstack'])
 
 # Generate minimum dependencies
 extras_require['tests-min'] = [dep.replace('>=', '==') for dep in extras_require['tests']

tox.ini

@@ -24,13 +24,14 @@ description =
 envdir =
     py37-unitbase: {toxworkdir}/py37-tests-base
     py37-min: {toxworkdir}/py37-tests-min
-    py37-!unitbase-!min: {toxworkdir}/py37-tests
+    py37-!unitbase-!min-!prospector: {toxworkdir}/py37-tests
     py38-unitbase: {toxworkdir}/py38-tests-base
     py38-min: {toxworkdir}/py38-tests-min
-    py38-!unitbase-!min: {toxworkdir}/py38-tests
+    py38-!unitbase-!min-!prospector: {toxworkdir}/py38-tests
     py39-unitbase: {toxworkdir}/py39-tests-base
     py39-min: {toxworkdir}/py39-tests-min
-    py39-!unitbase-!min: {toxworkdir}/py39-tests
+    py39-!unitbase-!min-!prospector: {toxworkdir}/py39-tests
+    prospector: {toxworkdir}/{envname}
 commands =
     flake8: flake8 setup.py cumin doc
     mypy: mypy cumin/
@@ -51,7 +52,8 @@ deps =
     # Use install_requires and the additional extras_require[NAME] from setup.py
     unitbase: .[tests-base]
     min: .[tests-min]
-    !min-!unitbase: .[tests]
+    prospector: .[prospector]
+    !min-!unitbase-!prospector: .[tests]
 setenv =
     min: CUMIN_MIN_DEPS=1