Change METH_VARARGS to METH_O; much faster parsing 🎉😅

[movermeyer]

What are you trying to accomplish?

When I saw that cPython's 3.11 datetime.fromisoformat was outperforming ciso8601.parse_datetime (without time zone information), I went looking to see what they were doing differently.

Their methods were defined as METH_O (i.e., method with single positional parameter), while ours were using METH_VARARGS (i.e., method with multiple positional parameters). It turns out there is a large performance benefit when you:

a) No longer have to call PyArg_ParseTuple
b) Allow Python to optimize based on knowing there is just a single parameter

What approach did you choose and why?

I changed each of the parser methods from METH_VARGS to METH_O.
This changed their C signatures, and so I had to plumb through the changes.

What should reviewers focus on?

...

The impact of these changes

Embarrassingly faster parsing times:

Module	Python 3.11	Python 3.10	Python 3.9	Python 3.8	Python 3.7	Python 2.7	Relative Slowdown
ciso8601 (this PR)	84 nsec	109 nsec	103 nsec	102 nsec	107 nsec	122 nsec	N/A
datetime.fromisoformat	109 nsec	N/A	N/A	N/A	N/A	N/A	1.3x
ciso8601 v2.2.0	130 nsec	154 nsec	147 nsec	158 nsec	158 nsec	168 nsec	1.5x