core: make mypy happy about importing fatfs

core/src/trezor/__init__.py

@@ -1,4 +1,10 @@
 import trezorconfig as config  # noqa: F401
 import trezorio as io  # noqa: F401
 
-fatfs = io.fatfs
+if False:
+    import trezorio.fatfs as fatfs
+else:
+    # a bug in mypy causes a crash at _usage site_ of the following:
+    fatfs = io.fatfs
+    # hence the if False branch that does what mypy understands - but which doesn't
+    # actually work because `trezorio.fatfs` is not importable.