Replace the cgi module by vendoring used functions

Python 3.13 dropped the deprecated cgi module. Remove the module and
include the relevant function to parse the content header.

src/simple_openid_connect/utils.py

@@ -1,7 +1,8 @@
 """
 Internal utilities
 """
-import cgi
+
+from typing import Iterator
 
 from simple_openid_connect.exceptions import ValidationError
 
@@ -11,7 +12,7 @@ def is_application_json(content_type: str) -> bool:
     Whether the given content type is `application/json`.
     This is needed because mime types can contain additional options which are ignored here.
     """
-    main_type, _options = cgi.parse_header(content_type)
+    main_type = _parse_header(content_type)
     return main_type == "application/json"
 
 
@@ -25,3 +26,28 @@ def validate_that(condition: bool, msg: str) -> None:
     """
     if not condition:
         raise ValidationError(msg)
+
+
+def _parse_header(line: str) -> str:
+    """Parse a Content-type like header.
+
+    Return the main content-type.
+    """
+    # Adapted from https://github.com/python/cpython/blob/3.12/Lib/cgi.py#L238
+    # by adding type hints and removing the parsing of additional mime type options.
+    return _parseparam(";" + line).__next__()
+
+
+def _parseparam(s: str) -> Iterator[str]:
+    # Adapted from https://github.com/python/cpython/blob/3.12/Lib/cgi.py#L226
+    # by adding type hints.
+    while s[:1] == ";":
+        s = s[1:]
+        end = s.find(";")
+        while end > 0 and (s.count('"', 0, end) - s.count('\\"', 0, end)) % 2:
+            end = s.find(";", end + 1)
+        if end < 0:
+            end = len(s)
+        f = s[:end]
+        yield f.strip()
+        s = s[end:]