Parallelize the test runs via pytest-xdist

Resolves #2458

pytest.ini

@@ -4,6 +4,9 @@ addopts=
 	--doctest-modules
 	--doctest-glob=pkg_resources/api_tests.txt
 	-r sxX
+
+	# `pytest-xdist`:
+	-n auto
 doctest_optionflags=ALLOW_UNICODE ELLIPSIS
 # workaround for warning pytest-dev/pytest#6178
 junit_family=xunit2

setup.cfg

@@ -58,6 +58,7 @@ testing =
     paver
     pip>=19.1 # For proper file:// URLs support.
     jaraco.envs
+    pytest-xdist
 
 docs =
     # Keep these in sync with docs/requirements.txt