BUG: round() on tz-aware DateTimeIndex (spanning DST switch) throws AmbiguousTimeError as if tz-unaware
- [X] I have checked that this issue has not already been reported.

- [X] 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
dt = pd.Timestamp("2020-10-25 02:00:00").tz_localize("Europe/Brussels", ambiguous=False).round('min')
```
```log
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslibs/timestamps.pyx", line 489, in pandas._libs.tslibs.timestamps.Timestamp.round
  File "pandas/_libs/tslibs/timestamps.pyx", line 443, in pandas._libs.tslibs.timestamps.Timestamp._round
  File "pandas/_libs/tslibs/timestamps.pyx", line 826, in pandas._libs.tslibs.timestamps.Timestamp.tz_localize
  File "pandas/_libs/tslibs/tzconversion.pyx", line 232, in pandas._libs.tslibs.tzconversion.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from 2020-10-25 02:00:00, try using the 'ambiguous' argument
```
```python
import pandas as pd
dt = pd.Timestamp("2020-10-25 02:00:00").tz_localize("Europe/Brussels", ambiguous=True).round('min')
```

```log
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslibs/timestamps.pyx", line 489, in pandas._libs.tslibs.timestamps.Timestamp.round
  File "pandas/_libs/tslibs/timestamps.pyx", line 443, in pandas._libs.tslibs.timestamps.Timestamp._round
  File "pandas/_libs/tslibs/timestamps.pyx", line 826, in pandas._libs.tslibs.timestamps.Timestamp.tz_localize
  File "pandas/_libs/tslibs/tzconversion.pyx", line 232, in pandas._libs.tslibs.tzconversion.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from 2020-10-25 02:00:00, try using the 'ambiguous' argument
```

#### Problem description

A localized Timestamp does not give an error when rounding.
A Series of a localized DateTimeIndex shouldn't either.
 
[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

#### Expected Output

Output is expected to be identical as for

```python
pandas.Timestamp("2020-10-25 02:00:00+01:00").round('min')
```
respectively 
```python
pandas.Timestamp("2020-10-25 02:00:0+02:00").round('min')
```

i.e. without AmbiguousTimeError

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

<details>

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]

</details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.9.1-050901-generic
Version          : #202010170731 SMP Sat Oct 17 07:42:59 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.3
numpy            : 1.19.3
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 44.0.0

rest is not installed
