Replace usage of cgi.parse_header with email.message.Message

CHANGELOG.rst

@@ -8,7 +8,8 @@ Version 0.10
 - Add ``NOTICE.txt`` to ``license_files``, #58
 - Use default SSL context when downloading classifiers from PyPI, #57
 - Remove ``setup.py``, #52
-- Explicitly limit oldest supported Python version.
+- Explicitly limit oldest supported Python version
+- Replace usage of ``cgi.parse_header`` with ``email.message.Message``
 
 Version 0.9
 ===========

src/validate_pyproject/formats.py

@@ -111,16 +111,16 @@ def pep517_backend_reference(value: str) -> bool:
 
 
 def _download_classifiers() -> str:
-    import cgi
     import ssl
+    from email.message import Message
     from urllib.request import urlopen
 
     url = "https://pypi.org/pypi?:action=list_classifiers"
     context = ssl.create_default_context()
     with urlopen(url, context=context) as response:
-        content_type = response.getheader("content-type", "text/plain")
-        encoding = cgi.parse_header(content_type)[1].get("charset", "utf-8")
-        return response.read().decode(encoding)
+        headers = Message()
+        headers["content_type"] = response.getheader("content-type", "text/plain")
+        return response.read().decode(headers.get_param("charset", "utf-8"))
 
 
 class _TroveClassifier: