Do not fail on unhandled ImportErrors

In gh-121 we realized that some people are ignoring documentation and
attempting to use adapters that only work with more recent versions of
Requests and urllib3. As such, we need to handle ImportErrors from
our compatibility module better and issue warnings closer to where a
user might be encountering the problem.

Closes gh-121

requests_toolbelt/_compat.py

@@ -14,16 +14,26 @@
 import requests
 
 try:
-    from requests.packages.urllib3 import connection
     from requests.packages.urllib3 import fields
     from requests.packages.urllib3 import filepost
     from requests.packages.urllib3 import poolmanager
 except ImportError:
-    from urllib3 import connection
     from urllib3 import fields
     from urllib3 import filepost
     from urllib3 import poolmanager
 
+try:
+    from requests.packages.urllib3.connection import HTTPConnection
+    from requests.packages.urllib3 import connection
+except ImportError:
+    try:
+        from urllib3.connection import HTTPConnection
+        from urllib3 import connection
+    except ImportError:
+        HTTPConnection = None
+        connection = None
+
+
 if requests.__build__ < 0x020300:
     timeout = None
 else:

requests_toolbelt/adapters/socket_options.py

@@ -1,12 +1,14 @@
 # -*- coding: utf-8 -*-
 """The implementation of the SocketOptionsAdapter."""
 import socket
+import warnings
 
 import requests
 from requests import adapters
 
 from .._compat import connection
 from .._compat import poolmanager
+from .. import exceptions as exc
 
 
 class SocketOptionsAdapter(adapters.HTTPAdapter):
@@ -33,11 +35,19 @@ class SocketOptionsAdapter(adapters.HTTPAdapter):
 
     """
 
-    default_options = getattr(
-        connection.HTTPConnection,
-        'default_socket_options',
-        [(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)]
-    )
+    if connection is not None:
+        default_options = getattr(
+            connection.HTTPConnection,
+            'default_socket_options',
+            [(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)]
+        )
+    else:
+        default_options = []
+        warnings.warn(exc.RequestsVersionTooOld,
+                      "This version of Requests is only compatible with a "
+                      "version of urllib3 which is too old to support "
+                      "setting options on a socket. This adapter is "
+                      "functionally useless.")
 
     def __init__(self, **kwargs):
         self.socket_options = kwargs.pop('socket_options',

requests_toolbelt/exceptions.py

@@ -14,3 +14,12 @@ class VersionMismatchError(Exception):
     appropriately but the version installed is not sufficient.
     """
     pass
+
+
+class RequestsVersionTooOld(Warning):
+    """Used to indiciate that the Requests version is too old.
+
+    If the version of Requests is too old to support a feature, we will issue
+    this warning to the user.
+    """
+    pass