Add attributes for pickling

When using the multiprocessing module, it throws an AttributeError,
complaining that the object does not have the attribute used. This adds
the missing attributes and allows them to be pickled.
Signed-off-by: An Ha <zero579911@gmail.com>

docker/api/client.py

@@ -83,6 +83,12 @@ class APIClient(
             configuration.
         user_agent (str): Set a custom user agent for requests to the server.
     """
+
+    __attrs__ = requests.Session.__attrs__ + ['_auth_configs',
+                                              '_version',
+                                              'base_url',
+                                              'timeout']
+
     def __init__(self, base_url=None, version=None,
                  timeout=DEFAULT_TIMEOUT_SECONDS, tls=False,
                  user_agent=DEFAULT_USER_AGENT, num_pools=DEFAULT_NUM_POOLS):

docker/transport/npipeconn.py

@@ -69,6 +69,11 @@ class NpipeHTTPConnectionPool(urllib3.connectionpool.HTTPConnectionPool):
 
 
 class NpipeAdapter(requests.adapters.HTTPAdapter):
+
+    __attrs__ = requests.adapters.HTTPAdapter.__attrs__ + ['npipe_path',
+                                                           'pools',
+                                                           'timeout']
+
     def __init__(self, base_url, timeout=60,
                  pool_connections=constants.DEFAULT_NUM_POOLS):
         self.npipe_path = base_url.replace('npipe://', '')

docker/transport/ssladapter.py

@@ -25,6 +25,10 @@ if sys.version_info[0] < 3 or sys.version_info[1] < 5:
 class SSLAdapter(HTTPAdapter):
     '''An HTTPS Transport Adapter that uses an arbitrary SSL version.'''
 
+    __attrs__ = HTTPAdapter.__attrs__ + ['assert_fingerprint',
+                                         'assert_hostname',
+                                         'ssl_version']
+
     def __init__(self, ssl_version=None, assert_hostname=None,
                  assert_fingerprint=None, **kwargs):
         self.ssl_version = ssl_version

docker/transport/unixconn.py

@@ -50,6 +50,11 @@ class UnixHTTPConnectionPool(urllib3.connectionpool.HTTPConnectionPool):
 
 
 class UnixAdapter(requests.adapters.HTTPAdapter):
+
+    __attrs__ = requests.adapters.HTTPAdapter.__attrs__ + ['pools',
+                                                           'socket_path',
+                                                           'timeout']
+
     def __init__(self, socket_url, timeout=60,
                  pool_connections=constants.DEFAULT_NUM_POOLS):
         socket_path = socket_url.replace('http+unix://', '')