bbox_inches=tight support for *all* figure artists.
Review actions.
Fixes #1862: The PS backend would inadvertently close file handles passed to it on Python 3.  The TextIOWrapper that handles the encoding from unicode to ascii needs to be detached before its destructor is called, otherwise it will call close on the underlying file object.  Also cleans up how with statements are handled to avoid the sometimes problematic null contextmanager.
Merge pull request #1862 from mdboom/ps-file-closing

Matplotlib savefig() closes BytesIO object when saving in postscript format
fix the pyplot version of rc_context

Before this, when using the rc_context dangling from pyplot, one would
get an AttributeError for ``__exit__``, because plt.rc_context wasn't
actually returning anything before this change.
Merge pull request #1866 from ivanov/fix-rc_context

fix the pyplot version of rc_context
Merge pull request #1448 from pelson/bbox_tight_all_artists

```bbox_inches="tight"``` support for *all* figure artists.