MAINT: port numpy#13188 for np_datetime simplification

[jbrockmendel]

our _libs/tslibs/src/np_datetime.c is based off of the numpy version. A recent PR there made a pretty nice simplfication to the relevant code. We should do the same.

https://github.com/numpy/numpy/pull/13188/files

[xcz011]

If possible, I’d like to contribute this. I may need some help since this is my first contribution. Thanks!

[jbrockmendel]

Great, a PR is welcome. If you're comfortable with C, it's just a matter of making sure we haven't changed anything that would make that patch inapplicable, then copy/paste, with appropriate attribution.