vfs: suppress some pytype errors around us using a private attribute
Looking at threading._MainThread seems like we're probably a little
unsupported, but since this code appears to work on both Python 2 and
3 I'm not going to sweat this for now.
Differential Revision: https://phab.mercurial-scm.org/D7282
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod