[BUG] Groupby on `DatetimeIndex` with `freq` doesn't preserve it

**Describe the bug**
When performing a groupby on a dataframe with `DatetimeIndex` that has `freq` set, doesn't retain it in the result.
This is the cause for failures in: `pandas-testing/pandas-tests/tests/resample/test_base.py`

**Steps/Code to reproduce bug**
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**Expected behavior**
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**Environment overview (please complete the following information)**
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**Environment details**
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**Additional context**
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