[3.13] gh-102555: Fix comment parsing in HTMLParser according to the HTML5 standard (GH-135664) (GH-136272)

* "--!>" now ends the comment.
* "-- >" no longer ends the comment.
* Support abnormally ended empty comments "<-->" and "<--->".

---------
(cherry picked from commit 8ac7613dc8)

Co-author: Kerim Kabirov <the.privat33r+gh@pm.me>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Lib/html/parser.py

@@ -29,7 +29,8 @@ attr_charref = re.compile(r'&(#[0-9]+|#[xX][0-9a-fA-F]+|[a-zA-Z][a-zA-Z0-9]*)[;=
 starttagopen = re.compile('<[a-zA-Z]')
 endtagopen = re.compile('</[a-zA-Z]')
 piclose = re.compile('>')
-commentclose = re.compile(r'--\s*>')
+commentclose = re.compile(r'--!?>')
+commentabruptclose = re.compile(r'-?>')
 # Note:
 #  1) if you change tagfind/attrfind remember to update locatetagend too;
 #  2) if you change tagfind/attrfind and/or locatetagend the parser will
@@ -336,6 +337,21 @@ class HTMLParser(_markupbase.ParserBase):
         else:
             return self.parse_bogus_comment(i)
 
+    # Internal -- parse comment, return length or -1 if not terminated
+    # see https://html.spec.whatwg.org/multipage/parsing.html#comment-start-state
+    def parse_comment(self, i, report=True):
+        rawdata = self.rawdata
+        assert rawdata.startswith('<!--', i), 'unexpected call to parse_comment()'
+        match = commentclose.search(rawdata, i+4)
+        if not match:
+            match = commentabruptclose.match(rawdata, i+4)
+            if not match:
+                return -1
+        if report:
+            j = match.start()
+            self.handle_comment(rawdata[i+4: j])
+        return match.end()
+
     # Internal -- parse bogus comment, return length or -1 if not terminated
     # see https://html.spec.whatwg.org/multipage/parsing.html#bogus-comment-state
     def parse_bogus_comment(self, i, report=1):