[doc] new behavior in virtualenv since 3.26

[skeleton] use python3 flake8 In tox "flake8" environment, use python 3 virtualenv, so we can check for python3 incompatible syntax.

[skeleton] drop whitelist_externals for flake8 whitelist_externals makes sense when used with sitepackages=true. In this case it will always rely on $VIRTUAL_ENV/bin/flake8 so, the whitelist_externals has no effect.

Add changelog note for running tests with pytest-xdist

Make test database template creation concurrent build_db_cache() is used in tests to create test database templates, i.e. DEFAULT_EMPTY_DB_ID (which is __default_empty_db__) and custom template database using CubicwebTC test_db_id/pre_setup_database API. When running tests in parallel using multiple processes, build_db_cache() may try to build the same database twice. Avoid this by adding synchronisation of process by using a file lock. So when two processes require the same template database, one build the database and others wait it to be created. Use filelock (https://github.com/benediktschmitt/py-filelock) library to have a portable (unix / windows) way for handling locks. Also filelock is packaged in debian: https://packages.debian.org/source/python-filelock

Make test database cache lookup concurrent has_cache() is used to dertermine if the template database already exists or not. When running tests accross multiple process, has_cache() wasn't aware of databases created by other processes because it used a cache to generate the cache... Since the discovering discover_cached_db() time is quite reasonable (it's just about listing file in a directory), let re-generate the cache on cache miss.

Fix flake8 issues since release 3.6.0 Flake8 had a new release which raise new issues, namely: W504: line break after binary operator F841: local variable 'ex' is assigned to but never used W605: invalid escape sequence F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive) Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run flake8 tests.

Stabilize flake8 tests on 3.26 branch Use basepython=python2 or flake8 installed with python3 will raise "[F821] undefined name 'buffer'" Pin flake8 < 3.6 which has new checks that doesn't pass on our current code base.

Fix typo in RDFS URI

[doc] Do not rely on lgc's sphinx_ext This module is broken (getting an ImportError) and I don't want to fix it. The only usage we make of it is for the "autodocstring" directive, and I don't know what it does. Based on the context, replacing autodocstring with either automodule or autoclass and removing sphinx_ext from required Sphinx extensions.