Revert "PYTHON-1915: Prohibit copying ClientSession objects (#480)"

This reverts commit 959039b.

Author: ShaneHarvey

doc/contributors.rst

@@ -88,4 +88,3 @@ The following is a list of people who have contributed to
 - Terence Honles (terencehonles)
 - Paul Fisher (thetorpedodog)
 - Julius Park (juliusgeo)
-- Ishmum Jawad Khan (ishmum123)

pymongo/client_session.py

@@ -827,9 +827,6 @@ def _start_retryable_write(self):
         self._check_ended()
         self._server_session.inc_transaction_id()
 
-    def __copy__(self):
-        raise TypeError('A ClientSession cannot be copied, create a new session instead')
-
 
 class _ServerSession(object):
     def __init__(self, generation):

test/test_session.py

@@ -779,11 +779,6 @@ def drop_db():
 
         wait_until(drop_db, 'dropped database after w=0 writes')
 
-    def test_session_not_copyable(self):
-        client = self.client
-        with client.start_session() as s:
-            self.assertRaises(TypeError, lambda: copy.copy(s))
-
 
 class TestCausalConsistency(unittest.TestCase):