[FIX] requirements.txt: unpin pytz

On Debian based systems, the `tzdata` package is maintained to reflect changes
in timezones and there is no need to upgrade the `python3-tz` package.
On the other hand, for those who are using `pip` and thus our `requirements.txt`,
the package needs to be up to date. By unpinning it in the requirements.txt:

- new installations based on pip will be up to date
- older installations based on pip can easily upgrade
- debian based installations have to maintain the tzdata package
- mixed installs like on runbot will rely on Debian tzdata

closes odoo/odoo#117527

closes odoo/odoo#120084

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>