Inherit HTTPConnection through urllib3.connection, not httplib

By inheriting from `urllib3.connection.HTTPConnection` (that inherits
from `httplib.HTTPConnection` itself), we can adapt to the internal
changes in urllib3 2.0 that added a `request()` method that is
incompatible with httplib.HTTPConnection.request.

This fixes the incompatibility between urllib3 2.0 and requests 1.26+,
which was the first version that stopped vendoring urllib3.

Reference: docker/docker-py#3113 (comment)
Fixes: #70

requests_unixsocket/adapters.py

@@ -3,11 +3,6 @@
 from requests.adapters import HTTPAdapter
 from requests.compat import urlparse, unquote
 
-try:
-    import http.client as httplib
-except ImportError:
-    import httplib
-
 try:
     from requests.packages import urllib3
 except ImportError:
@@ -16,7 +11,7 @@
 
 # The following was adapted from some code from docker-py
 # https://github.com/docker/docker-py/blob/master/docker/transport/unixconn.py
-class UnixHTTPConnection(httplib.HTTPConnection, object):
+class UnixHTTPConnection(urllib3.connection.HTTPConnection, object):
 
     def __init__(self, unix_socket_url, timeout=60):
         """Create an HTTP connection to a unix domain socket