TST: Add doctests on cython files
Adding `--doctest-cython` to `ci/run_tests.sh` to cover doctests in cython files.

Lots of them are failing at the moment. But I think few of them (like now, today), can be skipped using `# doctest: +SKIP`.

Below is the current list of failures atm.
```python
===================================================================================== short test summary info =====================================================================================
FAILED pandas/_libs/hashtable.cpython-38-darwin.so::pandas._libs.hashtable.Int64Factorizer.factorize
FAILED pandas/_libs/hashtable.cpython-38-darwin.so::pandas._libs.hashtable.ObjectFactorizer.factorize
FAILED pandas/_libs/hashtable.cpython-38-darwin.so::pandas._libs.hashtable.__test__.Int64Factorizer.factorize (line 134)
FAILED pandas/_libs/hashtable.cpython-38-darwin.so::pandas._libs.hashtable.__test__.ObjectFactorizer.factorize (line 88)
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.__test__.infer_dtype (line 1299)
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.__test__.is_iterator (line 242)
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.__test__.is_list_like (line 1051)
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.__test__.is_scalar (line 149)
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.infer_dtype
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.is_iterator
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.is_list_like
FAILED pandas/_libs/lib.cpython-38-darwin.so::pandas._libs.lib.is_scalar
FAILED pandas/_libs/tslibs/dtypes.cpython-38-darwin.so::pandas._libs.tslibs.dtypes.Resolution.from_attrname
FAILED pandas/_libs/tslibs/dtypes.cpython-38-darwin.so::pandas._libs.tslibs.dtypes.Resolution.get_reso_from_freq
FAILED pandas/_libs/tslibs/dtypes.cpython-38-darwin.so::pandas._libs.tslibs.dtypes.__test__.Resolution.from_attrname (line 223)
FAILED pandas/_libs/tslibs/dtypes.cpython-38-darwin.so::pandas._libs.tslibs.dtypes.__test__.Resolution.get_reso_from_freq (line 238)
FAILED pandas/_libs/tslibs/nattype.cpython-38-darwin.so::pandas._libs.tslibs.nattype.NaTType.now
FAILED pandas/_libs/tslibs/nattype.cpython-38-darwin.so::pandas._libs.tslibs.nattype.NaTType.today
FAILED pandas/_libs/tslibs/nattype.cpython-38-darwin.so::pandas._libs.tslibs.nattype.NaTType.utcfromtimestamp
FAILED pandas/_libs/tslibs/nattype.cpython-38-darwin.so::pandas._libs.tslibs.nattype.NaTType.utcnow
FAILED pandas/_libs/tslibs/period.cpython-38-darwin.so::pandas._libs.tslibs.period._Period.strftime
FAILED pandas/_libs/tslibs/period.cpython-38-darwin.so::pandas._libs.tslibs.period.__test__._Period.strftime (line 2243)
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.Timestamp.now
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.Timestamp.today
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.Timestamp.utcfromtimestamp
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.Timestamp.utcnow
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps._Timestamp.to_period
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.__test__.Timestamp.now (line 1048)
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.__test__.Timestamp.today (line 1075)
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.__test__.Timestamp.utcfromtimestamp (line 1115)
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.__test__.Timestamp.utcnow (line 1101)
FAILED pandas/_libs/tslibs/timestamps.cpython-38-darwin.so::pandas._libs.tslibs.timestamps.__test__._Timestamp.to_period (line 905)
FAILED pandas/_libs/tslibs/timezones.cpython-38-darwin.so::pandas._libs.tslibs.timezones.__test__.tz_standardize (line 375)
FAILED pandas/_libs/tslibs/timezones.cpython-38-darwin.so::pandas._libs.tslibs.timezones.tz_standardize
================================================================================= 34 failed, 106 passed in 1.10s ==================================================================================