BUG: `tz_localize` drops `freq` from `DatetimIndex`
- [x] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd

s = pd.Series(
    [1, 2, 3, 4, 5], index=pd.date_range("2020", periods=5, freq="D")
)
print(s.index.freq)  # <Day>
print(s.tz_localize("europe/london").index.freq)  # None

```

#### Problem description

The index should retain the `freq` attribute despite localization.

#### Output of ``pd.show_versions()``

<details>

It raises
```python-traceback
ImportError: Can't determine version for hypothesis
```

</details>
