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.
Add requirements.txt files in test directories
These files, located in each test directory (when needed), list test
dependencies supplementary to install requirements.
Some requirements.txt files include psycopg2 because it won't get installed by
logilab-database apparently.
Related to #5447161.