setup: avoid setuptools 67 - it can't handle celery's broken pytz dependency

With setuptools 67 or later, launching Kallithea fails as:

$ gearbox serve -c my.ini --reload
15:56:54,111 ERROR [gearbox] Expected closing RIGHT_PARENTHESIS
pytz (>dev)
~^

The `packaging` vendored in setuptools cannot handle the broken syntax
`Requires-Dist: pytz (>dev)` in
venv/lib/python3.11/site-packages/celery-5.0.5.dist-info/METADATA .

The old celery version currently used by Kallithea is wrong, and setuptools has
moved on after a reasonable grace period. We thus have to work around and avoid
latest setuptools.

See https://github.com/pypa/setuptools/issues/3889 .

docs: always use dev_requirements.txt together with main version constraints in setup.py

pip isn't good at considering old constraints when installing or updating
packages later. Instead, install everything at once.

Note: pip and setuptools are usually installed automatically and could perhaps
be removed from documentation. But for now, let's keep it.

Jenkinsfile: run each py.test on a separate executor

Running all py.test processes in parallel on the same executor is not a good idea.
If a node has not much RAM, it could run out of memory when all py.test processes
run at the same time. And If there are only 2 CPU cores, it doesn't make sense
to run more then 2 processes either.
Therefore use a separate executor for each one. The py.test processes will thus
only run in parallel if there are multiple Jenkins executors available.

jenkinsfile: delete all old files in the workspace

Especial important when files have been renamed and there are *.pyc files
left over from the last build.

jenkinsfile: create venv in special folder instead of jenkins workspace

The path to a jenkins workspace tends to get very long. But the maximum for a
shebang line is usually 127 characters. This can cause problems with git hook
scripts or when calling pip. To avoid these problems create the virtualenv
in the a folder under $JENKINS_HOME which is usually much shorter.

jenkinsfile: remove unnecessary lines

We use TEST_DB environment variable instead of editing test.ini to change the DB
configuration.

tests: add Jenkinsfile for automatic creation of Jenkins projects for testing Kallithea itself

This is the new way of how to build and run tests with jenkins.
It runs pylint and py.test with 4 different settings:
1. default
2. with German language settings
3. with MySQL DB
4. with PostgreSQL DB

A major advantage of this new Jenkinsfile approach is that Jenkins will be able
to automatically create new projects for each branch (or pull request) that
contains a jenkinsfile. There is thus no need for maintaining multiple jenkins
projects.