logging: handle "cannot delete old logfile" error

E.g. on Windows, files open in one process cannot be deleted by another process.
With file logging enabled, if an old logfile was open in a text editor,
Electrum could crash during startup.

```
E | __main__ |
Traceback (most recent call last):
  File "...\electrum\run_electrum", line 391, in main
    handle_cmd(
  File "...\electrum\run_electrum", line 403, in handle_cmd
    configure_logging(config)
  File "...\electrum\electrum\logging.py", line 278, in configure_logging
    _configure_file_logging(log_directory)
  File "...\electrum\electrum\logging.py", line 107, in _configure_file_logging
    _delete_old_logs(log_directory)
  File "...\electrum\electrum\logging.py", line 98, in _delete_old_logs
    os.remove(str(f))
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: '...\\AppData\\Roaming\\Electrum\\testnet\\logs\\electrum_log_20210414T023751Z_25008.log'
```

electrum/logging.py

@@ -77,7 +77,10 @@ electrum_logger.setLevel(logging.DEBUG)
 def _delete_old_logs(path, keep=10):
     files = sorted(list(pathlib.Path(path).glob("electrum_log_*.log")), reverse=True)
     for f in files[keep:]:
-        os.remove(str(f))
+        try:
+            os.remove(str(f))
+        except OSError as e:
+            _logger.warning(f"cannot delete old logfile: {e}")
 
 
 _logfile_path = None