BUG: to_period() freq was not infered
- [x] closes #33358
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

---

When running ```pytest -q --cache-clear pandas/tests/ -k "period"``` some tests in ```pandas/tests/indexing/``` are failing, but when running ```pytest -q --cache-clear pandas/tests/indexing/``` __all__ the tests are passing (on my local machine), so I figured and debug from here, thoughts?
> thoughts?

we cant rely on grepping to get all the relevant tests.  i recommend `pytest pandas/tests --skip-slow --skip-db`, takes about 15 minutes for me.

Look at the failing tests, choose one and start executing it step-by-step in the REPL

```
off = "QS"
rng = pd.date_range("01-Jan-2012", periods=8, freq=off)

>>> prng = rng.to_period()   # <-- raises, so recurse into the `to_period` method
```
 looks good, can you add a whatsnew note in 1.1 in the datetime section for bug fixes. ping on green. ping @jreback  thanks @MomIsBestFriend 