TST: fixed cython doctests
- [x] closes #43757

Skipped some of the tests which were failing
thanks @MarcoGorelli for reviewing this!

- yes I think that's valid for cython docstrings as well. I have removed the `# doctest: +SKIP` for the line `pandas supports PEP 3141 numbers:` and it's running fine.

- I have tried my best and was able to do the first two examples, but not able to replicate the third one (I don't have much experience working with `tz`).

So currently, only two tests are being skipped, i.e. last example in `tz` and `Int64Factorizer.factorize` in hashtable.pyx

Edit - Another point worth highlighting, is the below is failing on my local (as the output is different than from the CI).
```
??? >>> pd.Timestamp.fromtimestamp(1584199972)
Expected:
    Timestamp('2020-03-14 15:32:52')
Got:
    Timestamp('2020-03-14 21:02:52') > Edit - Another point worth highlighting, is the below is failing on my local (as the output is different than from the CI). [...] `??? >>> pd.Timestamp.fromtimestamp(1584199972)`

This is going to depend on what timezone you are in locally. > > Edit - Another point worth highlighting, is the below is failing on my local (as the output is different than from the CI). [...] `??? >>> pd.Timestamp.fromtimestamp(1584199972)`
> 
> This is going to depend on what timezone you are in locally.

would be a good idea to skip this no? (or maybe make it timezone independent if possible, to ensure it doesn't fail in anyones local) IMO the 
```
In [33]: from dateutil.tz.tz import tzutc as tz

In [34]: tz
Out[34]: dateutil.tz.tz.tzutc

In [35]: from pandas._libs.tslibs.timezones import tz_standardize

In [36]: tz_standardize(tz)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-36-94d2ff882b92> in <module>
----> 1 tz_standardize(tz)

TypeError: Argument 'tz' has incorrect type (expected datetime.tzinfo, got _TzSingleton)
```
test can be removed, if it doesn't run.

Other than that, the rest looks good to me thanks @debnathshoham 

i suppose that if we had more doc-tests in cython this would pick them up. if someone wants to create an issue about adding more doc-strings wouldn't object (for example there is an older PR that adds a ton to the timestamp functions). 