Fix FIND_LINKS_REGEX to match any whitespace after <a

[danishashko]

The regex <a [^<>]+?> uses a literal space which misses valid anchor tags that use tabs or multiple spaces between <a and the first attribute.

Changing to <a\s+[^<>]+?> covers any whitespace character, which is more robust and consistent with how HTML is written in the wild.

Quick check:

import re
old = re.compile(r"<a [^<>]+?>", re.I)
new = re.compile(r"<a\s+[^<>]+?>", re.I)
print(bool(old.search('<a\thref="http://example.com">')))  # False
print(bool(new.search('<a\thref="http://example.com">')))  # True