contextlib: port ExitStack from CPython 3.4

[dxxb]

See discussion in #49

[dxxb]

I am not sure I like the fact I changed instances of of assertIsNone() to assert is None. I'm happy to add assertIsNone() to unittest and get back to this.

[pfalcon]

I am not sure I like the fact I changed instances of of assertIsNone() to assert is None. I'm happy to add assertIsNone() to unittest and get back to this.

Sure, feel free to, unittest is not size-critical (it's just big mess o'code from my point of view, so I didn't even try to port it, instead was adding method on as-needed basis, other contributed a bunch too).

[dxxb]

I'd rather use 'assert x is None' directly but it would result in more line changes when porting tests from CPython. I have created #52 and rebased this PR on it.

[dxxb]

Hi @pfalcon, this is ready to be reviewed.

[dxxb]

PR updated. Prefixed incompatible test names with underscore.

[pfalcon]

Merged, thanks.

[dxxb]

Apologies for the oversight and thank you.