Add tox.ini file
* Factor out `commands` for most environments using {envname} substitution to
build pytest commande.
* Positional arguments are passed on to `pytest` (others may also be passed
after a `--`).
* Set PATH env variable for tests requiring PostgreSQL.
* Also added a `doc` target to build the book.
Also update .hgignore exclude .tox directory (where virtualenvs will be
created and cubicweb.egg-info created by sdist command).
Closes #5447161.
