Test that deprecated Diff.renamed property warns

This starts on a test.deprecation subpackage for deprecation tests.
Having these tests in a separate directory inside the test suite
may or may not be how they will ultimately be orgnaized, but it has
two advantages:

- Once all the tests are written, it should be easy to see what in
  GitPython is deprecated.

- Some deprecation warnings -- those on module or class attribute
  access -- will require the introduction of new dynamic behavior,
  and thus run the risk of breaking static type checking. So that
  should be checked for, where applicable. But currently the test
  suite has no type annotations and is not checked by mypy. Having
  deprecation-related tests under the same path will make it easier
  to enable mypy for just this part of the test suite (for now).

It is also for this latter reason that the one test so far is
written without using the GitPython test suite's existing fixtures
whose uses are harder to annotate. This may be changed if
warranted, though some of the more complex deprecation-related
tests may benefit from being written as pure pytest tests.

Although a number of deprecated features in GitPython do already
issue warnings, Diff.renamed is one of the features that does not
yet do so. So the newly introduced test will fail until that is
fixed in the next commit.

Author: EliahKagan

test/deprecation/__init__.py

@@ -0,0 +1,2 @@
+# This module is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/

test/deprecation/test_various.py

@@ -0,0 +1,20 @@
+# This module is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
+
+"""Tests of assorted deprecation warnings with no extra subtleties to check."""
+
+from git.diff import NULL_TREE
+from git.repo import Repo
+
+import pytest
+
+
+def test_diff_renamed_warns(tmp_path):
+    (tmp_path / "a.txt").write_text("hello\n", encoding="utf-8")
+    repo = Repo.init(tmp_path)
+    repo.index.add(["a.txt"])
+    commit = repo.index.commit("Initial commit")
+    (diff,) = commit.diff(NULL_TREE)  # Exactly one file in the diff.
+
+    with pytest.deprecated_call():
+        diff.renamed