Advertise support for the string parser to also work on bytes.

Note that this already worked, except the type annotation was wrong
and there were no tests.

src/parsy/__init__.py

@@ -5,7 +5,7 @@
 import re
 from dataclasses import dataclass
 from functools import wraps
-from typing import Any, Callable, FrozenSet
+from typing import Any, AnyStr, Callable, FrozenSet
 
 __version__ = "2.1"
 
@@ -529,13 +529,16 @@ def fail(expected: str) -> Parser:
     return Parser(lambda _, index: Result.failure(index, expected))
 
 
-def string(expected_string: str, transform: Callable[[str], str] = noop) -> Parser:
+def string(expected_string: AnyStr, transform: Callable[[AnyStr], AnyStr] = noop) -> Parser:
     """
     Returns a parser that expects the ``expected_string`` and produces
     that string value.
 
     Optionally, a transform function can be passed, which will be used on both
     the expected string and tested string.
+
+    This parser can also be instantiated with a bytes value, in which case it can
+    should be applied to a stream of bytes.
     """
 
     slen = len(expected_string)

tests/test_parsy.py

@@ -32,13 +32,13 @@
 
 
 class TestParser(unittest.TestCase):
-    def test_string(self):
+    def test_string_str(self):
         parser = string("x")
         self.assertEqual(parser.parse("x"), "x")
 
         self.assertRaises(ParseError, parser.parse, "y")
 
-    def test_string_transform(self):
+    def test_string_transform_str(self):
         parser = string("x", transform=lambda s: s.lower())
         self.assertEqual(parser.parse("x"), "x")
         self.assertEqual(parser.parse("X"), "x")
@@ -53,6 +53,19 @@ def test_string_transform_2(self):
 
         self.assertRaises(ParseError, parser.parse, "dog")
 
+    def test_string_bytes(self):
+        parser = string(b"x")
+        self.assertEqual(parser.parse(b"x"), b"x")
+
+        self.assertRaises(ParseError, parser.parse, b"y")
+
+    def test_string_transform_bytes(self):
+        parser = string(b"x", transform=lambda s: s.lower())
+        self.assertEqual(parser.parse(b"x"), b"x")
+        self.assertEqual(parser.parse(b"X"), b"x")
+
+        self.assertRaises(ParseError, parser.parse, b"y")
+
     def test_regex_str(self):
         parser = regex(r"[0-9]")