Fix _Timestamp edge cases (#534)

* Add failing test cases for timestamp conversion

* Fix timestamp to datetime conversion

* Fix formatting

* Move timestamp tests outside of inputs folder

---------

Co-authored-by: Lukas Bindreiter <lukas.bindreiter@tilebox.io>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

tests/test_timestamp.py

@@ -0,0 +1,27 @@
+from datetime import (
+    datetime,
+    timezone,
+)
+
+import pytest
+
+from betterproto import _Timestamp
+
+
+@pytest.mark.parametrize(
+    "dt",
+    [
+        datetime(2023, 10, 11, 9, 41, 12, tzinfo=timezone.utc),
+        datetime.now(timezone.utc),
+        # potential issue with floating point precision:
+        datetime(2242, 12, 31, 23, 0, 0, 1, tzinfo=timezone.utc),
+        # potential issue with negative timestamps:
+        datetime(1969, 12, 31, 23, 0, 0, 1, tzinfo=timezone.utc),
+    ],
+)
+def test_timestamp_to_datetime_and_back(dt: datetime):
+    """
+    Make sure converting a datetime to a protobuf timestamp message
+    and then back again ends up with the same datetime.
+    """
+    assert _Timestamp.from_datetime(dt).to_datetime() == dt