Be explicit about pickle protocol and encoding (#24454)

On Python3 and Python2 use pickle slightly differently so we need to be
explicit about some things.

If pickles could be shared between python2 and python3, as in
ansible-connection and the pickle cache, we need to specify the protocol
to use when dumping and the encoding to use for byte strings when
loading.

The dumping protocol needs to be no higher than 2 as python-2 only
supports up to protocol 2.  The encoding should usually be 'bytes' so
that python2 str type becomes python3 bytes type.  However, doing this
means that we must make sure that the objects being serialized properly
make their strings into text strings except when they're supposed to be
bytes.  If strings are improperly byte strings, they may cause
tracebacks on the receiving end

bin/ansible-connection

@@ -41,8 +41,6 @@ import syslog
 import datetime
 import logging
 
-from io import BytesIO
-
 from ansible import constants as C
 from ansible.module_utils._text import to_bytes, to_native
 from ansible.module_utils.six import PY3
@@ -214,7 +212,10 @@ class Server():
                             display.display("socket operation is CONTEXT", log_only=True)
                             pc_data = data.split(b'CONTEXT: ', 1)[1]
 
-                            pc_data = cPickle.loads(pc_data)
+                            if PY3:
+                                pc_data = cPickle.loads(pc_data, encoding='bytes')
+                            else:
+                                pc_data = cPickle.loads(pc_data)
 
                             pc = PlayContext()
                             pc.deserialize(pc_data)
@@ -267,11 +268,14 @@ def main():
         cur_line = stdin.readline()
         init_data = b''
         while cur_line.strip() != b'#END_INIT#':
-            if cur_line  == b'':
+            if cur_line == b'':
                 raise Exception("EOF found before init data was complete")
             init_data += cur_line
             cur_line = stdin.readline()
-        pc_data = cPickle.loads(init_data)
+        if PY3:
+            pc_data = cPickle.loads(init_data, encoding='bytes')
+        else:
+            pc_data = cPickle.loads(init_data)
 
         pc = PlayContext()
         pc.deserialize(pc_data)