Don't require prevtx for coins with BIP-143 fork id (#352)

Coins like Bitcoin Cash and Bitcoin Gold always use BIP-143 signatures
that do not require previous transactions.

trezorlib/btc.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the License along with this library.
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
-from . import messages
+from . import coins, messages
 from .tools import CallException, expect, normalize_nfc, session
 
 
@@ -91,6 +91,11 @@ def sign_tx(client, coin_name, inputs, outputs, details=None, prev_txes=None):
     # set up a transactions dict
     txes = {None: messages.TransactionType(inputs=inputs, outputs=outputs)}
     # preload all relevant transactions ahead of time
+    if coin_name in coins.by_name:
+        load_prevtxes = not coins.by_name[coin_name]["force_bip143"]
+    else:
+        load_prevtxes = True
+    if load_prevtxes:
         for inp in inputs:
             if inp.script_type not in (
                 messages.InputScriptType.SPENDP2SHWITNESS,