Add datetime.fromisoformat to the benchmarks

[Morwenn]

Python 3.7 added datetime.fromisoformat to the datetime library, which might be another target to add to the benchmarks without timezones for completeness.

Now a few remarks about the function:

• It is only meant to parse the result of isoformat, so it isn't super flexible compared to what you provide
• I looked at the implementation and while it should be faster than using the equivalent strptime, it is still implemented in pure Python, so it's highly unlikely to beat your implementation.

[movermeyer]

Thanks! Yes, I've been looking into datetime.fromisoformat intensely.

In cPython, it is actually implemented both in Python and in C and the C version is faster than any other method (including ciso8601).
I've been working on a backport of it to older Python versions.

There is an open issue to rewrite the docs to be more comprehensive and inclusive of alternatives.

[Morwenn]

I wasn't aware that there was also a C implementation, it's nice to know :)