Update request urllib3 (#464)

* requests: updated to version 2.27.1

* urllib3: updated to version 1.26.18

lib/urllib3/_collections.py

@@ -17,9 +17,10 @@ except ImportError:  # Platform-specific: No threads available
 
 
 from collections import OrderedDict
-from .exceptions import InvalidHeader
-from .packages.six import iterkeys, itervalues, PY3
 
+from .exceptions import InvalidHeader
+from .packages import six
+from .packages.six import iterkeys, itervalues
 
 __all__ = ["RecentlyUsedContainer", "HTTPHeaderDict"]
 
@@ -174,7 +175,7 @@ class HTTPHeaderDict(MutableMapping):
     def __ne__(self, other):
         return not self.__eq__(other)
 
-    if not PY3:  # Python 2
+    if six.PY2:  # Python 2
         iterkeys = MutableMapping.iterkeys
         itervalues = MutableMapping.itervalues
 
@@ -190,7 +191,7 @@ class HTTPHeaderDict(MutableMapping):
 
     def pop(self, key, default=__marker):
         """D.pop(k[,d]) -> v, remove specified key and return the corresponding value.
-          If key is not found, d is returned if given, otherwise KeyError is raised.
+        If key is not found, d is returned if given, otherwise KeyError is raised.
         """
         # Using the MutableMapping function directly fails due to the private marker.
         # Using ordinary dict.pop would expose the internal structures.
@@ -267,6 +268,24 @@ class HTTPHeaderDict(MutableMapping):
         else:
             return vals[1:]
 
+    def _prepare_for_method_change(self):
+        """
+        Remove content-specific header fields before changing the request
+        method to GET or HEAD according to RFC 9110, Section 15.4.
+        """
+        content_specific_headers = [
+            "Content-Encoding",
+            "Content-Language",
+            "Content-Location",
+            "Content-Type",
+            "Content-Length",
+            "Digest",
+            "Last-Modified",
+        ]
+        for header in content_specific_headers:
+            self.discard(header)
+        return self
+
     # Backwards compatibility for httplib
     getheaders = getlist
     getallmatchingheaders = getlist