GH9570 allow timedelta string conversion without leading zero

[chrisgilmerproj]

See #9570

This commit allows for the Timedelta class to take a construction like Timedelta('0:00:00') or Timedelta('00:00:00').

[chrisgilmerproj]

Test: https://travis-ci.org/chrisgilmerproj/pandas/builds/58278744

[TomAugspurger]

Could you add a release note in doc/source/whatsnew/v0.16.1?

[chrisgilmerproj]

Yep, will do!

[chrisgilmerproj]

Test: https://travis-ci.org/chrisgilmerproj/pandas/builds/58287006

[chrisgilmerproj]

This now supports the following:

In [1]: from pandas import Timedelta; Timedelta('-01:00:00')
Out[1]: Timedelta('-1 days +23:00:00')

In [2]: from pandas import Timedelta; Timedelta('-1:00:00')
Out[2]: Timedelta('-1 days +23:00:00')

In [3]: from pandas import Timedelta; Timedelta('00:00:00')
Out[3]: Timedelta('0 days 00:00:00')

In [4]: from pandas import Timedelta; Timedelta('0:00:00')
Out[4]: Timedelta('0 days 00:00:00')

[chrisgilmerproj]

All right, tests have passed @TomAugspurger @jreback

[TomAugspurger]

Looks good to me. I'll let @jreback handle the merges today (unless he says otherwise).