Fix python 3.12 deprecation warning (#3751)

* Fix python 3.12 deprecation warning

utcfromtimestamp is deprecated replace
with fromtimestamp with explict UTC timezone

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

* add changelog for 3751

Author: jenshnielsen

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix python 3.12 deprecation warning
+  ([#3751](https://github.com/open-telemetry/opentelemetry-python/pull/3751))
+
 ## Version 1.23.0/0.44b0 (2024-02-23)
 
 - Use Attribute rather than boundattribute in logrecord

opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.py

@@ -23,7 +23,9 @@
 
 def ns_to_iso_str(nanoseconds):
     """Get an ISO 8601 string from time_ns value."""
-    ts = datetime.datetime.utcfromtimestamp(nanoseconds / 1e9)
+    ts = datetime.datetime.fromtimestamp(
+        nanoseconds / 1e9, tz=datetime.timezone.utc
+    )
     return ts.strftime("%Y-%m-%dT%H:%M:%S.%fZ")