py3: use io.BytesIO directly Previously, pycompat.bytesio and pycompat.stringio referred to io.BytesIO. And util.bytesio and util.stringio aliased the pycompat symbols. This commit switches everything to use io.BytesIO directly. util.bytesio and util.stringio still exist to provide backwards compatibility, as they were the preferred symbols. Differential Revision: https://phab.mercurial-scm.org/D12252

check-py3-compat: drop support for Python 2 We no longer support Python 2 so we can drop support for linting code for Python 2 support. This gets rid of the check for `from __future__`, enabling us to delete those imports. Differential Revision: https://phab.mercurial-scm.org/D12251