pyln-testing: suppress mypy on FileLock

3.0.12 here:
```
pyln/testing/utils.py:9: error: Cannot find implementation or library stub for module named 'filelock'
pyln/testing/utils.py:9: error: Cannot find implementation or library stub for module named 'filelock'
pyln/testing/utils.py:9: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-testing/pyln/testing/utils.py

@@ -6,7 +6,7 @@ from pyln.testing.btcproxy import BitcoinRpcProxy
 from collections import OrderedDict
 from decimal import Decimal
 from ephemeral_port_reserve import reserve  # type: ignore
-from filelock import FileLock
+from filelock import FileLock  # type: ignore
 from pyln.client import LightningRpc
 from pyln.client import Millisatoshi