Use timezone aware datetime

The official Python docs recommend explicitly passing a timezone object
instead of using `utcnow` or `utcfromtimestamp`:

https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow
https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp

Author: abravalheri

src/setuptools_scm/version.py

@@ -1,6 +1,5 @@
 import os
 import re
-import time
 import warnings
 from datetime import datetime
 from datetime import timezone
@@ -119,9 +118,11 @@ def __init__(
         self.distance = distance
         self.node = node
         self.node_date = node_date
-        self.time = datetime.utcfromtimestamp(
-            int(os.environ.get("SOURCE_DATE_EPOCH", time.time()))
-        )
+        if "SOURCE_DATE_EPOCH" in os.environ:
+            date_epoch = int(os.environ["SOURCE_DATE_EPOCH"])
+            self.time = datetime.fromtimestamp(date_epoch, timezone.utc)
+        else:
+            self.time = datetime.now(timezone.utc)
         self._extra = kw
         self.dirty = dirty
         self.preformatted = preformatted

testing/test_git.py

@@ -2,6 +2,7 @@
 import sys
 from datetime import date
 from datetime import datetime
+from datetime import timezone
 from os.path import join as opj
 from unittest.mock import Mock
 from unittest.mock import patch
@@ -215,7 +216,7 @@ def test_git_dirty_notag(today, wd, monkeypatch):
     assert wd.version.startswith("0.1.dev1")
     if today:
         # the date on the tag is in UTC
-        tag = datetime.utcnow().date().strftime(".d%Y%m%d")
+        tag = datetime.now(timezone.utc).date().strftime(".d%Y%m%d")
     else:
         tag = ".d20090213"
     # we are dirty, check for the tag