Kiva fonttool weight constants don't match OpenType and CSS specs

[corranwebster]

The names of the weights are defined here: https://docs.microsoft.com/en-us/typography/opentype/spec/os2#usweightclass

But these don't match the names defined here in all cases:

enable/kiva/fonttools/_constants.py

Lines 90 to 105 in 718c863

	weight_dict = {
	"ultralight": 100,
	"light": 200,
	"normal": 400,
	"regular": 400,
	"book": 400,
	"medium": 500,
	"roman": 500,
	"semibold": 600,
	"demibold": 600,
	"demi": 600,
	"bold": 700,
	"heavy": 800,
	"extra bold": 800,
	"black": 900,
	}

For example "light" is 300 in both specs, but 200 in the constants file.

This is likely to result in mis-matches in font queries because FontEntry objects have the wrong value.

Fixing this may require bumping the font database version and re-generating the font database.

There is a potential tension here because the constants are used for scoring closeness of match, but are also used for interpreting the contents of font files.

[corranwebster]

There is one fortunate feature that the font weights are not being used heavily by the higher-level code right now - the only values that the querying code sees are likely "normal" and "bold" which are mapped correctly.

[corranwebster]

Also, this is entirely backend code - these names are used for matching font names to font queries, but font queries are not really end-user exposed, so I think that updating this list to be more inclusive and correct is fine. It should silently make font matches better where it matters, and won't break end-user code.

The only possible end-user breakage is if there is something that was matching before and now doesn't, but because only "bold" and "normal" are really exposed, we should be fine on that.