some clean-ups now that we require python 3.8

In particular, asyncio.CancelledError no longer inherits Exception (it inherits BaseException directly)

electrum/exchange_rate.py

@@ -80,9 +80,6 @@ class ExchangeBase(Logger):
             self.logger.info(f"getting fx quotes for {ccy}")
             self.quotes = await self.get_rates(ccy)
             self.logger.info("received fx quotes")
-        except asyncio.CancelledError:
-            # CancelledError must be passed-through for cancellation to work
-            raise
         except aiohttp.ClientError as e:
             self.logger.info(f"failed fx quotes: {repr(e)}")
             self.quotes = {}