BUG: Odd timezone offset change with old datetimes with tz_convert
- [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
[
    pd.to_datetime(["1900-01-01"]).tz_localize("UTC").tz_convert("Australia/Broken_Hill"),
    pd.to_datetime(["1900-01-01"]).tz_localize("UTC").tz_convert("Pacific/Marquesas"),
    pd.to_datetime(["2000-01-01"]).tz_localize("UTC").tz_convert("Pacific/Marquesas"),
    pd.to_datetime(["2000-01-01"]).tz_localize("UTC").tz_convert("Australia/Broken_Hill"),
    pd.to_datetime(["1900-01-01"]).tz_localize("UTC").tz_convert("Etc/GMT-9"),
    pd.to_datetime(["2000-01-01"]).tz_localize("UTC").tz_convert("Etc/GMT-9"),
]
```
Returns:
```
[DatetimeIndex(['1900-01-01 09:26:00+09:26'], dtype='datetime64[ns, Australia/Broken_Hill]', freq=None),
 DatetimeIndex(['1899-12-31 14:42:00-09:18'], dtype='datetime64[ns, Pacific/Marquesas]', freq=None),
 DatetimeIndex(['1999-12-31 14:30:00-09:30'], dtype='datetime64[ns, Pacific/Marquesas]', freq=None),
 DatetimeIndex(['2000-01-01 10:30:00+10:30'], dtype='datetime64[ns, Australia/Broken_Hill]', freq=None),
 DatetimeIndex(['1900-01-01 09:00:00+09:00'], dtype='datetime64[ns, Etc/GMT-9]', freq=None),
 DatetimeIndex(['2000-01-01 09:00:00+09:00'], dtype='datetime64[ns, Etc/GMT-9]', freq=None)]
```

#### Problem description

Timezone offset seems to change when it shouldn't? I'm not 100% this is not the correct behavior but it seems odd.
I hope this is known behavior and not something esoteric.

#### Expected Output

Probably this:
```
[DatetimeIndex(['1900-01-01 10:30:00+10:30'], dtype='datetime64[ns, Australia/Broken_Hill]', freq=None),
 DatetimeIndex(['1899-12-31 14:30:00-09:30'], dtype='datetime64[ns, Pacific/Marquesas]', freq=None),
 DatetimeIndex(['1999-12-31 14:30:00-09:30'], dtype='datetime64[ns, Pacific/Marquesas]', freq=None),
 DatetimeIndex(['2000-01-01 10:30:00+10:30'], dtype='datetime64[ns, Australia/Broken_Hill]', freq=None),
 DatetimeIndex(['1900-01-01 09:00:00+09:00'], dtype='datetime64[ns, Etc/GMT-9]', freq=None),
 DatetimeIndex(['2000-01-01 09:00:00+09:00'], dtype='datetime64[ns, Etc/GMT-9]', freq=None)]
```

I'm not sure these timezones even existed then so this might be an invalid calculation.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.7.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-73-lowlatency
Version          : #82-Ubuntu SMP PREEMPT Wed Apr 14 19:19:50 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.19.1
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 50.3.0.post20201006
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 1.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.19
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : None

</details>
