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.

Spelling fixes in comments and docstrings

[web/views] Clean up confusing backwards compatibility code Dead code since fbb370195199, related to #4943392.

[server/test] avoid lgdb warnings from str to unicode conversion

[web/test] Modernize unittest_propertysheet.py - remove call to os.makedirs in global scope - fold CACHEDIR as an instance attribute - bonus: use unittest directly instead of lgc.testlib

[web/webconfig] Drop pre-3.9 backward compatibility method "has_resource"

[web] Stop using negative indices with os.stat results The python doc says: More items may be added at the end by some implementations. .st_mtime is also more readable than [-2].

[doc] small rst fixes and cleanups in changelog files Related to #4832808

[doc] make the changelog chapter a one html page Related to #4832808