worker: silence type error when calling pickle pytype is complaining that the argument to `pickle.load()` is not an `IO`. pytype isn't wrong: `_blockingreader` doesn't implement `io.RawIOBase`, only `read()` and `readline()`. But it appears this is enough for pickle. So we silence the false positive. This fixes a regression introduced by D12304 / cc0e059d2af8: worker: remove Python 2 support code. Differential Revision: https://phab.mercurial-scm.org/D12337