Clarify that the Headers class is a Sequence

This is both in the docs (via the changelog) and by inheriting from
the Sequence ABC.

Author: pgjones

h11/_headers.py

@@ -1,4 +1,5 @@
 import re
+from collections.abc import Sequence
 
 from ._abnf import field_name, field_value
 from ._util import bytesify, LocalProtocolError, validate
@@ -62,7 +63,7 @@
 _field_value_re = re.compile(field_value.encode("ascii"))
 
 
-class Headers:
+class Headers(Sequence):
     """
     A list-like interface that allows iterating over headers as byte-pairs
     of (lowercased-name, value).

newsfragments/112.doc.rst

@@ -0,0 +1,3 @@
+Clarify that the Headers class is a Sequence and inherit from the
+collections Sequence abstract base class to also indicate this (and
+gain the mixin methods). See also #104.