Ignore leading whitespace when checking for HTML doctype

[domdfcoding]

Ref #10825 (comment)

[malkir]

lgtm

[domdfcoding]

I'm not sure about the CI failures. They're getting AttributeError: 'int' object has no attribute 'startswith' from within setuptools.

[uranusjr]

Hmm, I read the HTML5 spec and it seems we are still missing a lot of logic. This PR stands on its own so we should still merge it (unless you want to tackle them in this PR), but hopefully someone can come in and fix those other issues.

[notatallshaw]

I'm not sure about the CI failures. They're getting AttributeError: 'int' object has no attribute 'startswith' from within setuptools.

Yes, seems tests are broken on Ubuntu at the moment, I am getting the same error for something completely unrelated: https://github.com/pypa/pip/runs/5010487348?check_suite_focus=true

[pradyunsg]

I see @domdfcoding has filed pypa/setuptools#3063 already! :)

This concern has been addressed!

[pradyunsg]

LGTM!

[pfmoore]

hopefully someone can come in and fix those other issues.

I'm willing to take a look at this (I was intending to, in fact) but my time till the end of February is somewhat limited, so if someone else wants to get to it first that's fine.

[pradyunsg]

In #10868, I'm proposing we drop this fallback I added in the spirit of expidiecy yesterday -- I think there's a much better behaviour here that I didn't think of in the heat of the moment yesterday.