Backport PR #43467 on branch 1.3.x (Make group_mean compatible with NaT)
Backport PR #43467: Make group_mean compatible with NaT
arm failure (circle CI)

```
=========================== short test summary info ============================
FAILED pandas/tests/frame/methods/test_rename.py::TestRename::test_rename_inplace
= 1 failed, 158075 passed, 16037 skipped, 860 xfailed, 1 xpassed, 2 warnings in 1740.54s (0:29:00) =
```

This is a known flaky test.