Fix tox configuration breaking on spaces in the project path

The {envpython} substitution is not quoted, so spaces in the path are
treated as argument separators and cause the test runs to fail.
To work around this, we now always use an unqualified python command
instead of the {envpython} substitution. This is safe because the tox
commands are always run in a virtual environment, so the python command
is guaranteed to point to the environment's Python and not the system
default.

tox.ini

@@ -3,7 +3,7 @@
 envlist = py{36,37,38},flake8,mypy
 
 [testenv]
-commands = {envpython} -m unittest discover --start-directory ./tests
+commands = python -m unittest discover --start-directory ./tests
 
 [testenv:flake8]
 deps =