[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#120155

closes odoo/odoo#120164

X-original-commit: 710a2b2a
Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>

requirements.txt

@@ -46,7 +46,7 @@ pyserial==3.5
 python-dateutil==2.8.1
 python-ldap==3.4.0 ; sys_platform != 'win32'  # min version = 3.2.0 (Focal with security backports)
 python-stdnum==1.16
-pytz==2021.1
+pytz  # no version pinning to avoid OS perturbations
 pyusb==1.0.2 ; python_version <= '3.10'
 pyusb==1.2.1 ; python_version > '3.10'
 qrcode==6.1