exchange_rate: fix regenerating currencies.json

```
cannot find currencies.json. will regenerate it now.
Traceback (most recent call last):
  File "...\electrum\run_electrum", line 94, in <module>
    from electrum.logging import get_logger, configure_logging  # import logging submodule first
  File "...\electrum\electrum\__init__.py", line 27, in <module>
    from . import daemon
  File "...\electrum\electrum\daemon.py", line 55, in <module>
    from .exchange_rate import FxThread
  File "...\electrum\electrum\exchange_rate.py", line 539, in <module>
    CURRENCIES = get_exchanges_and_currencies()
  File "...\electrum\electrum\exchange_rate.py", line 529, in get_exchanges_and_currencies
    loop = util.get_asyncio_loop()
  File "...\electrum\electrum\util.py", line 1578, in get_asyncio_loop
    raise Exception("event loop not created yet")
Exception: event loop not created yet
```

electrum/exchange_rate.py

@@ -526,11 +526,14 @@ def get_exchanges_and_currencies():
                 for name, klass in exchanges.items():
                     exchange = klass(None, None)
                     await group.spawn(get_currencies_safe(name, exchange))
-    loop = util.get_asyncio_loop()
+
+    loop = asyncio.new_event_loop()
     try:
         loop.run_until_complete(query_all_exchanges_for_their_ccys_over_network())
     except Exception as e:
         pass
+    finally:
+        loop.close()
     with open(path, 'w', encoding='utf-8') as f:
         f.write(json.dumps(d, indent=4, sort_keys=True))
     return d