Make test_contrasts.py ignore token.Whitespace

[not-my-profile]

With the VisibleWhitespaceFilter Pygments can be configured to replace
whitespace characters like spaces, tabs and newlines with Unicode
characters like ·, » and ¶ respectively.

The min_contrast metric is meant to represent the readability of a
style. Increasing it should mean that the style becomes more readable.

This commit therefore excludes the token.Whitespace contrast from the
min_contrast calculation, since giving whitespace characters a high
contrast would make them so intrusive that they would actually hinder
the readability of the source code.

Screenshots for illustration

Current default style:

WCAG AA would look like (which IMO is clearly less readable):

[Anteru]

Good catch. Another good reason to be more precise about whitespace handling, but that is in @blu-base capable hands.

[not-my-profile]

Thanks :) Though I don't quite get your comment on whitespace precision.

[blu-base]

In many lexers whitespaces are detected as the Text token, not the Text.Whitespace token.
It seems this has historical reasons, since the Whitespace token has only been introduced in the later development.
For this reason, there are many lexers which cannot make use of the VisibleWhitespaceFilter, yet - since there are no Whitespace token declared, or only some.

I started to review most of them in order to fix that inhomogenity. Though i just scratched the surface (#1905)