PYTHON-2605: Improve mongodb+srv:// error message when dnspython is not installed (#602)

(cherry picked from commit 5388fde)

Author: Kn99HN

pymongo/uri_parser.py

@@ -16,6 +16,7 @@
 """Tools to parse and validate a MongoDB URI."""
 import re
 import warnings
+import sys
 
 from bson.py3compat import string_type, PY3
 
@@ -425,8 +426,12 @@ def parse_uri(uri, default_port=DEFAULT_PORT, validate=True, warn=False,
         scheme_free = uri[SCHEME_LEN:]
     elif uri.startswith(SRV_SCHEME):
         if not _HAVE_DNSPYTHON:
-            raise ConfigurationError('The "dnspython" module must be '
-                                     'installed to use mongodb+srv:// URIs')
+            python_path = sys.executable or "python"
+            raise ConfigurationError(
+                'The "dnspython" module must be '
+                'installed to use mongodb+srv:// URIs. '
+                'To fix this error install pymongo with the srv extra:\n '
+                '%s -m pip install "pymongo[srv]"' % (python_path))
         is_srv = True
         scheme_free = uri[SRV_SCHEME_LEN:]
     else: