tox: Run everything but unit tests under Python 3

The demise of Python 2.7 is not far off and we intend to drop support
for it in Sphinx 2.0. As a result, there probably isn't a better time
than now to start testing with Python 3 by default. There are a couple
of advantages to this:

- flake8 is stricter under Python 3 and we don't need to ignore files
  with Python 3 syntax
- We get to dogfood Sphinx against its own documentation using Python 3
- pylint gains support for Python 3 syntax

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -27,12 +27,14 @@ commands=
     pytest -Wall --durations 25 {posargs}
 
 [testenv:flake8]
+basepython = python3
 description =
     Run style checks.
 commands =
     flake8
 
 [testenv:pylint]
+basepython = python3
 description =
     Run source code analyzer.
 deps =
@@ -42,6 +44,7 @@ commands =
     pylint --rcfile utils/pylintrc sphinx
 
 [testenv:coverage]
+basepython = python3
 description =
     Run code coverage checks.
 setenv =
@@ -51,6 +54,7 @@ commands =
     coverage report
 
 [testenv:mypy]
+basepython = python3
 description =
     Run type checks.
 deps =
@@ -59,6 +63,7 @@ commands=
     mypy sphinx/
 
 [testenv:docs]
+basepython = python3
 description =
     Build documentation.
 commands =