contract_wrappers.py: don't auto-import wrappers

This was discovered while minimizing CircleCI steps to dianose a problem
with running the Launch Kit Backend in CircleCI.

These classes should be imported via the
zero_ex.contract_wrappers.exchange and
zero_ex.contract_wrappers.erc20_token modules, respectively.  We
permitted importing them from just zero_ex.contract_wrappers back when
they were the only wrappers we had, but now that we have so many
different contracts being wrapped, this is just another list to keep
manually updated (which, obviously is error prone, since it slipped
through the cracks already), so it's better to just not support this
type of import.

python-packages/contract_wrappers/src/zero_ex/contract_wrappers/__init__.py

@@ -161,7 +161,7 @@
 fill.  This example fills the order completely, but partial fills are possible
 too.
 
->>> from zero_ex.contract_wrappers import Exchange
+>>> from zero_ex.contract_wrappers.exchange import Exchange
 >>> exchange = Exchange(
 ...     provider=ganache,
 ...     contract_address=NETWORK_TO_ADDRESSES[NetworkId.GANACHE].exchange,
@@ -324,5 +324,3 @@
 """
 
 from .tx_params import TxParams
-from .erc20_token import ERC20Token
-from .exchange import Exchange