Implement zoneinfo support to drop dependency to pytz

Zoneinfo was introduced within python 3.9.

The support of pytz will be removed within RHEL 10 [1].

2023.2 (bobcat) will move our testing runtime to py3.9 and py3.10
so we want to see pytz removed within this series.

tzdata is required at runtime in our gates, because, by default,
zoneinfo uses the system’s time zone data if available; if no system
time zone data is available, the library will fall back to using the
first-party tzdata package available on PyPI. Apparently our gates have no
time zone data available nor tzdata installed by default because we get the
following error without tzdata installed [3]:
`ModuleNotFoundError: No module named 'tzdata'

So I prefer to add tzdata in our requirements to avoid runtime failure
related to time zone and ensure that time zone are always available.

[1] https://issues.redhat.com/browse/RHEL-219
[2] https://review.opendev.org/c/openstack/governance/+/872232
[3]
https://zuul.opendev.org/t/openstack/build/0a1576775e894b09bc31269fea00ba03/log/job-output.txt#1445`

Depends-on: https://review.opendev.org/c/openstack/requirements/+/875854
Change-Id: I8d87d54f6f5ded8caee6cb780bacb39afea0fea1

4 files changed, 21 insertions, 13 deletions

diff --git a/oslo_serialization/msgpackutils.py b/oslo_serialization/msgpackutils.py
index ff7b267..01003bf 100644
--- a/oslo_serialization/msgpackutils.py
+++ b/oslo_serialization/msgpackutils.py
@@ -33,10 +33,10 @@ import functools
 import itertools
 import uuid
 from xmlrpc import client as xmlrpclib
+import zoneinfo
 
 import msgpack
 from oslo_utils import importutils
-from pytz import timezone
 
 netaddr = importutils.try_import("netaddr")
 
@@ -236,7 +236,7 @@ class DateTimeHandler(object):
             'microsecond': dt.microsecond,
         }
         if dt.tzinfo:
-            tz = dt.tzinfo.tzname(None)
+            tz = str(dt.tzinfo)
             dct['tz'] = tz
         return dumps(dct, registry=self._registry)
 
@@ -263,9 +263,9 @@ class DateTimeHandler(object):
                                minute=dct['minute'],
                                second=dct['second'],
                                microsecond=dct['microsecond'])
-        if 'tz' in dct:
-            tzinfo = timezone(dct['tz'])
-            dt = tzinfo.localize(dt)
+        if 'tz' in dct and dct['tz']:
+            tzinfo = zoneinfo.ZoneInfo(dct['tz'])
+            dt = dt.replace(tzinfo=tzinfo)
         return dt
 
 
diff --git a/oslo_serialization/tests/test_msgpackutils.py b/oslo_serialization/tests/test_msgpackutils.py
index a8a91e9..3acc70d 100644
--- a/oslo_serialization/tests/test_msgpackutils.py
+++ b/oslo_serialization/tests/test_msgpackutils.py
@@ -15,10 +15,10 @@
 import datetime
 import itertools
 from xmlrpc import client as xmlrpclib
+import zoneinfo
 
 import netaddr
 from oslotest import base as test_base
-from pytz import timezone
 
 from oslo_serialization import msgpackutils
 from oslo_utils import uuidutils
@@ -145,20 +145,22 @@ class MsgPackUtilsTest(test_base.BaseTestCase):
         self.assertEqual(today, _dumps_loads(today))
 
     def test_datetime_tz_clone(self):
-        eastern = timezone('US/Eastern')
+        eastern = zoneinfo.ZoneInfo('US/Eastern')
         now = datetime.datetime.now()
-        e_dt = eastern.localize(now)
+        e_dt = now.replace(tzinfo=eastern)
         e_dt2 = _dumps_loads(e_dt)
         self.assertEqual(e_dt, e_dt2)
         self.assertEqual(e_dt.strftime(_TZ_FMT), e_dt2.strftime(_TZ_FMT))
 
     def test_datetime_tz_different(self):
-        eastern = timezone('US/Eastern')
-        pacific = timezone('US/Pacific')
+        eastern = zoneinfo.ZoneInfo('US/Eastern')
+        pacific = zoneinfo.ZoneInfo('US/Pacific')
         now = datetime.datetime.now()
 
-        e_dt = eastern.localize(now)
-        p_dt = pacific.localize(now)
+        now = now.replace(tzinfo=eastern)
+        e_dt = now
+        now = now.replace(tzinfo=pacific)
+        p_dt = now
 
         self.assertNotEqual(e_dt, p_dt)
         self.assertNotEqual(e_dt.strftime(_TZ_FMT), p_dt.strftime(_TZ_FMT))
diff --git a/releasenotes/notes/implement-zoneinfo-to-remove-pytz-c136b33bbfbfe59f.yaml b/releasenotes/notes/implement-zoneinfo-to-remove-pytz-c136b33bbfbfe59f.yaml
new file mode 100644
index 0000000..24c7d7f
--- /dev/null
+++ b/releasenotes/notes/implement-zoneinfo-to-remove-pytz-c136b33bbfbfe59f.yaml
@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Implement zoneinfo to allow us to remove pytz's dependency. zoneinfo
+    was introduced by python 3.9, and the series 2023.2 (bobcat) set py39
+    as the minimal supported runtime, so we are able to remove pytz.
diff --git a/requirements.txt b/requirements.txt
index d8c62c4..d31beef 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,4 +10,4 @@
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 msgpack>=0.5.2 # Apache-2.0
 oslo.utils>=3.33.0 # Apache-2.0
-pytz>=2013.6 # MIT
+tzdata>=2022.4 # MIT