bpo-38216, bpo-36274: Allow subclasses to separately override validation and encoding behavior (GH-16448)

* bpo-38216: Allow bypassing input validation

* bpo-36274: Also allow the URL encoding to be overridden.

* bpo-38216, bpo-36274: Add tests demonstrating a hook for overriding validation, test demonstrating override encoding, and a test to capture expectation of the interface for the URL.

* Call with skip_host to avoid tripping on the host checking in the URL.

* Remove obsolete comment.

* Make _prepare_path_encoding its own attr.

This makes overriding just that simpler.

Also, don't use the := operator to make backporting easier.

* Add a news entry.

* _prepare_path_encoding -> _encode_prepared_path()

* Once again separate the path validation and request encoding, drastically simplifying the behavior. Drop the guarantee that all processing happens in _prepare_path.

Lib/http/client.py

@@ -1085,18 +1085,15 @@ class HTTPConnection:
         else:
             raise CannotSendRequest(self.__state)
 
-        # Save the method we use, we need it later in the response phase
+        # Save the method for use later in the response phase
         self._method = method
-        if not url:
-            url = '/'
-        # Prevent CVE-2019-9740.
-        if match := _contains_disallowed_url_pchar_re.search(url):
-            raise InvalidURL(f"URL can't contain control characters. {url!r} "
-                             f"(found at least {match.group()!r})")
+
+        url = url or '/'
+        self._validate_path(url)
+
         request = '%s %s %s' % (method, url, self._http_vsn_str)
 
-        # Non-ASCII characters should have been eliminated earlier
-        self._output(request.encode('ascii'))
+        self._output(self._encode_request(request))
 
         if self._http_vsn == 11:
             # Issue some standard headers for better HTTP/1.1 compliance
@@ -1174,6 +1171,18 @@ class HTTPConnection:
             # For HTTP/1.0, the server will assume "not chunked"
             pass
 
+    def _encode_request(self, request):
+        # ASCII also helps prevent CVE-2019-9740.
+        return request.encode('ascii')
+
+    def _validate_path(self, url):
+        """Validate a url for putrequest."""
+        # Prevent CVE-2019-9740.
+        match = _contains_disallowed_url_pchar_re.search(url)
+        if match:
+            raise InvalidURL(f"URL can't contain control characters. {url!r} "
+                             f"(found at least {match.group()!r})")
+
     def putheader(self, header, *values):
         """Send a request header line to the server.