FIX: Pip 20+ support and fix tests

Apparently, the tests did not run since the
testenv did not match, so the builds were green
no matter the code.

I changed the tox.ini and added py38-pip20 and
made sure it failed before fixing the imports
in pkgversion.py.

On another note, we should probably not use
internals like this because it will randomly
break (like now). Fixing it now so we are
unblocked at least.

Makefile

@@ -29,7 +29,7 @@ clean: pyclean docsclean
 	@rm -rf venv
 
 venv:
-	@python3.6 -m venv venv
+	@python3.8 -m venv venv
 	@$(PIP) install -U "pip>=7.0" -q
 	@$(PIP) install -r $(DEPS)
 

pkgversion/pkgversion.py

@@ -5,9 +5,13 @@
 from subprocess import PIPE, Popen
 
 try:
-    from pip._internal.download import PipSession
-    from pip._internal.req import parse_requirements
-except ImportError:
+    try:  # pip20+
+        from pip._internal.network.session import PipSession
+        from pip._internal.req import parse_requirements
+    except:  # pip10+
+        from pip._internal.download import PipSession
+        from pip._internal.req import parse_requirements
+except ImportError:  # pip<10
     # Output expected ImportErrors for PIP < 10.
     from pip.download import PipSession
     from pip.req import parse_requirements

tox.ini

@@ -6,6 +6,7 @@ envlist =
     py27-{pip9,pip10}
     py35-{pip9,pip10}
     py36-{pip9,pip10}
+    py38-pip20
     cov
     isort-check
     isort-fix
@@ -14,68 +15,53 @@ envlist =
 
 skipsdist = true
 
+[testenv]
 basepython =
-    py27-{pip9,pip10}: python2.7
-    py35-{pip9,pip10}: python3.5
-    py36-{pip9,pip10}: python3.6
-    cov: python3.6
+    py27: python2.7
+    py35: python3.5
+    py36: python3.6
+    py38: python3.8
+    cov: python3.8
 deps =
     -rrequirements/requirements-base.txt
     -rrequirements/requirements-testing.txt
+    pip9: pip<10.0
+    pip10: pip<11.0
+    pip20: pip<21.0
+commands =
+     py.test tests
 
 [testenv:cov]
-basepython= python3.6
+basepython= python3.8
 commands =
     py.test tests --cov-report=term-missing --cov-append --cov=pkgversion --cov-report xml {posargs}
 deps =
     -rrequirements/requirements-base.txt
     -rrequirements/requirements-testing.txt
 
-[general]
-commands =
-     py.test tests
-deps =
-    -rrequirements/requirements-base.txt
-    -rrequirements/requirements-testing.txt
-
-[testenv:py27-{pip9,pip10}]
-basepython = python2.7
-commands =
-    {[general]commands}
-
-[testenv:py35-{pip9,pip10}]
-basepython = python3.5
-commands =
-   {[general]commands}
-
-[testenv:py36-{pip9,pip10}]
-basepython = python3.6
-commands =
-    {[general]commands}
-
 ##
 # Flake8 linting
 ##
 
 [testenv:lint]
-basepython = python3.6
+basepython = python3.8
 commands = flake8 pkgversion tests --exclude=pkgversion/__init__.py
 deps =
     flake8
 
 [testenv:docs]
-basepython = python3.6
+basepython = python3.8
 commands = sphinx-build -W -b html {toxinidir}/docs {toxinidir}/docs/_build/html
 deps =
     -rrequirements/requirements-testing.txt
 
 [testenv:isort-check]
-basepython = python3.6
+basepython = python3.8
 commands = isort -rc -c pkgversion tests
 deps = isort
 
 [testenv:isort-fix]
-basepython = python3.6
+basepython = python3.8
 commands = isort -rc pkgversion tests
 deps = isort