venv: limit pip version to stay below 24.1

Latest pip version pip fail:
WARNING: Ignoring version 5.0.5 of celery since it has invalid metadata:
Requested celery<5.1,>=5 from .../celery-5.0.5-py3-none-any.whl (from Kallithea==0.7.0) has invalid metadata: Expected matching RIGHT_PARENTHESIS for LEFT_PARENTHESIS, after version specifier
pytz (>dev)
~^
Please use pip<24.1 if you need to use this version.

We already use setuptools<67 for the same reason.

Pip will keep noting that a newer pip version is available. Resist the
temptation to upgrade.

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 .

py3: officially support Python 3

All tests pass and no known regressions.

py3: switch to use Python 3 interpreter, temporarily leaving many things very broken until they have been migrated/fixed in a reviewable way

Bump Mercurial minimum version to 5.2 - the first version that claim stable py3
support.

scripts: handle "Python 2.7 reached the end of its life" message

The script failed with:

Error: pip detected following problems:
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support

scripts: new maintainer script validate-minimum-dependency-versions

Automate what we can.

This script could be added later to an encompassing script
'prepare-for-release' (which would also do other release-related steps).

The trick '2> >(tee "$log" >&2)' sends the error output to the console while
at the same time writing to a file. It uses bash-specific syntax (process
substitution: >(...) )
See https://stackoverflow.com/a/692407/2941347 .

Includes contributions by Mads Kiilerich.