Add DeprecationWarnings to inform users of plans

After a long discussion in IRC and on several issues, the developers of
requests have decided to remove specific functions from requests.utils
in version 3.0.0. To give users ample time to prepare for this, we've
added DeprecationWarnings long in advance. See also the planning of this
in issue psf#2266.

requests/utils.py

@@ -19,6 +19,7 @@
 import sys
 import socket
 import struct
+import warnings
 
 from . import __version__
 from . import certs
@@ -287,6 +288,11 @@ def get_encodings_from_content(content):
 
     :param content: bytestring to extract encodings from.
     """
+    warnings.warn((
+        'In requests 3.0, get_encodings_from_content will be removed. For '
+        'more information, please see the discussion on issue #2266. (This'
+        ' warning should only appear once.)'),
+        DeprecationWarning)
 
     charset_re = re.compile(r'<meta.*?charset=["\']*(.+?)["\'>]', flags=re.I)
     pragma_re = re.compile(r'<meta.*?content=["\']*;?charset=(.+?)["\'>]', flags=re.I)
@@ -354,6 +360,11 @@ def get_unicode_from_response(r):
     2. fall back and replace all unicode characters
 
     """
+    warnings.warn((
+        'In requests 3.0, get_unicode_from_response will be removed. For '
+        'more information, please see the discussion on issue #2266. (This'
+        ' warning should only appear once.)'),
+        DeprecationWarning)
 
     tried_encodings = []