Use string serialization to serialize url()

That’s what’s proposed by CSSOM:
https://www.w3.org/TR/2021/WD-cssom-1-20210826/#serialize-a-url

tinycss2/serializer.py

@@ -84,23 +84,6 @@ def serialize_string_value(value):
     return _re_string_value.sub(_serialize_string_value_match, value)
 
 
-def serialize_url(value):
-    return ''.join(
-        r"\'" if c == "'" else
-        r'\"' if c == '"' else
-        r'\\' if c == '\\' else
-        r'\ ' if c == ' ' else
-        r'\9 ' if c == '\t' else
-        r'\A ' if c == '\n' else
-        r'\D ' if c == '\r' else
-        r'\C ' if c == '\f' else
-        r'\(' if c == '(' else
-        r'\)' if c == ')' else
-        c
-        for c in value
-    )
-
-
 # https://drafts.csswg.org/css-syntax/#serialization-tables
 def _serialize_to(nodes, write):
     """Serialize an iterable of nodes to CSS syntax.

tinycss2/tokenizer.py

@@ -8,7 +8,7 @@
     HashToken, IdentToken, LiteralToken, NumberToken, ParenthesesBlock, ParseError,
     PercentageToken, SquareBracketsBlock, StringToken, UnicodeRangeToken, URLToken,
     WhitespaceToken)
-from .serializer import serialize_string_value, serialize_url
+from .serializer import serialize_string_value
 
 _NUMBER_RE = re.compile(r'[-+]?([0-9]*\.)?[0-9]+([eE][+-]?[0-9]+)?')
 _HEX_ESCAPE_RE = re.compile(r'([0-9A-Fa-f]{1,6})[ \n\t]?')
@@ -77,7 +77,7 @@ def parse_component_value_list(css, skip_comments=False):
                 if url_pos >= length or css[url_pos] not in ('"', "'"):
                     value, pos, error = _consume_url(css, pos)
                     if value is not None:
-                        repr = 'url({})'.format(serialize_url(value))
+                        repr = 'url("{}")'.format(serialize_string_value(value))
                         if error is not None:
                             error_key = error[0]
                             if error_key == 'eof-in-string':