mark/expression: allow backslash characters in identifiers

Fixes #8983.

src/_pytest/mark/expression.py

@@ -6,7 +6,7 @@ expression: expr? EOF
 expr:       and_expr ('or' and_expr)*
 and_expr:   not_expr ('and' not_expr)*
 not_expr:   'not' not_expr | '(' expr ')' | ident
-ident:      (\w|:|\+|-|\.|\[|\])+
+ident:      (\w|:|\+|-|\.|\[|\]|\\)+
 
 The semantics are:
 
@@ -88,7 +88,7 @@ class Scanner:
                 yield Token(TokenType.RPAREN, ")", pos)
                 pos += 1
             else:
-                match = re.match(r"(:?\w|:|\+|-|\.|\[|\])+", input[pos:])
+                match = re.match(r"(:?\w|:|\+|-|\.|\[|\]|\\)+", input[pos:])
                 if match:
                     value = match.group(0)
                     if value == "or":