CI: Setup 3.9 Travis Build
- [x] closes https://github.com/pandas-dev/pandas/issues/33395

- In a very similar fashion to @jbrockmendel work for 3.8 - might be useful to have start a 3.9 build so we can start eliminating the issues @TomAugspurger mentioned in https://github.com/pandas-dev/pandas/issues/33395.

This is using currently using `Python 3.9.0a5+`

Currently failing with
```
ImportError while loading conftest '/home/travis/build/pandas-dev/pandas/pandas/conftest.py'.
pandas/conftest.py:1123: in <module>
    ("period", [pd.Period(2013), pd.NaT, pd.Period(2018)]),
pandas/_libs/tslibs/period.pyx:2470: in pandas._libs.tslibs.period.Period.__new__
    freq = Resolution.get_freq(reso)
pandas/_libs/tslibs/resolution.pyx:240: in pandas._libs.tslibs.resolution.Resolution.get_freq
    return cls._reso_freq_map[resostr]
E   AttributeError: type object 'type' has no attribute '_reso_freq_map'
The command "ci/run_tests.sh" exited with 4.
cache.2
store build cache
```