Use upstream dockerpty 0.3.2

This reverts commit 60411e9f05ac2b9245053491cef7c3cda9e923f1.

Closes #556

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Conflicts:
	requirements.txt
	setup.py
Don't select stdin when interactive=False

Patch from https://github.com/d11wtq/dockerpty/pull/24

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #588 from bfirsh/use-upstream-dockerpty

Use upstream dockerpty 0.3.2