Rewrite common executables to use the Python selected by tox

In https://github.com/jaraco/skeleton/issues/86, @mtelka proposes to replace `pytest` in the tox `commands` with `python -m pytest` in order to avoid ambiguity in which Python version is used to execute the tests.

As described in that issue, making that change has other effects and would exist only to support a narrow set of uses.

Since `tox-current-env` is implicated in the failure and since it already re-writes the handling of `python` as an executable, perhaps this project should also handle other common executables (pytest to start, but maybe others) by replacing such invocations with `python -m pytest` or perhaps `{sys.executable} $(which pytest)`.

This approach would address the issue more selectively and surgically where it's needed and avoid the need to adapt many (all?) downstream packages to use a non-standard approach to accommodate users of this project.