Revert "Update stub for builtin `open` function due to PEP-519."
Revert "Update stub for builtin `open` function due to PEP-519."

This reverts commit 48b271c862283273c4047ff8776c56ffa5646160.

See https://github.com/python/typeshed/pull/991#issuecomment-288160993
for details; basically this disturbed mypy's bootstrap import cycle.

li-dan/typeshed,azag0/typeshed,euresti/typeshed