follow-up prev: some clean-ups

re https://github.com/spesmilo/electrum/pull/7492

electrum/invoices.py

@@ -136,7 +136,7 @@ class OnchainInvoice(Invoice):
             if not (0 <= value <= TOTAL_COIN_SUPPLY_LIMIT_IN_BTC * COIN):
                 raise InvoiceError(f"amount is out-of-bounds: {value!r} sat")
         elif isinstance(value, str):
-            if value != "!":
+            if value != '!':
                 raise InvoiceError(f"unexpected amount: {value!r}")
         else:
             raise InvoiceError(f"unexpected amount: {value!r}")