CLN: removed unused import
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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Not 100% sure about this, can close anytime if not a good PR
I also benched marked it:

master:

```
In [1]: import pandas as pd                                                                                   

In [2]: %timeit pd.Period("2000-01-01", freq="D")                                                             
165 µs ± 777 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)
```

---

PR:

```
In [1]: import pandas as pd                                                                                   

In [2]: %timeit pd.Period("2000-01-01", freq="D")                                                             
166 µs ± 449 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)
``` Why is isort not complaining with the original code? Is it validating that file? > Why is isort not complaining with the original code? Is it validating that file?

@datapythonista ```isort``` is not checking that file ATM, in the release of ```isort``` 5.0.0 it will begin to support ```.pyx``` files.

I think you ment ```flake8```, and I don't know why it's not catching it (or if it's even checking```.pyx``` files) @datapythonista @jbrockmendel any outstanding objections? lgtm