Remove messytables dependency from xlsx import

[yohanboniface]

Next step after #265.

⚠️ Work in progress ⚠️

Some points to notice, and to be discussed/arbitrated:

• I've removed complex() call from _is_numeric helper, as complex("j") returns True, and I'm not exactly sure how to work around this and how much it's important to recognize a complex number here

With the complex check, this table headers will not be recognized (because j is a valid complex value):

a	b	c	d	e	f	g	h	i	j
9:14 PM	9/19/2019	1:10	10:20:30	100:20:30	vendredi 19 janvier 1900	7/4/1776	7/4/1976	samedi 30 décembre 1899	#FMT

• I've ported from messytable the idea of trying to select only rows enough cells as header candidate (instead of the blindly first row with at least one non empty cell; this is the behaviour we had until now anyway, because we were using headers_guess from messytable, and not our own version.

With our version this table:

Title
A	B	C
1	4	7
2	5	8
3	6	9

Will be imported with Title,, as headers, instead of A,B,C.

• test_excel_single_merged_cell output table is not respected, and I'm not sure to understand exactly if the first structure was explicitly expected or not, so I'll need some lights here

Here is the fixture table:

Before:

After:

Thanks in advance for your feedback!

cc @alexmojaki

[alexmojaki]

Thanks @yohanboniface, this looks great, and I'm happy with your decisions. Is it still a work in progress?

[yohanboniface]

Ah, sorry, well, it's a work in progress in the sense that it needs a review and certainly some arbitration :)

[alexmojaki]

So can I merge it?

[yohanboniface]

Yes, if you agree with the points I mentioned in the description! :)

[alexmojaki]

Thank you!