add Context manager and default context under threading

[thisiscam]

This implements https://github.com/inducer/islpy/issues/22

Semantics of context pushing is added as a test case.

With this change:

• DEFAULT_CONTEXT is deprecated (with a warning) in favor of get_default_context()
• get_default_context() always returns a thread unique default context for the first call in a thread.
• push_context(ctx=none) can be used to push new default context(s), which allows for custom default Context.
• Context now implements __copy__ and __deepcopy__ which always returns the identity.

[thisiscam]

I have put TODO_VERSION everywhere in the docs. Please take a look and change them to something appropriate?

[inducer]

Update islpy/__init__.py

Please have proper commit messages.

[inducer]

(OK to leave these, just for the future.)

[thisiscam]

Sorry, I believe they were generated automatically by GitHub. I will definitely note this in the future.

[inducer]

Sorry, I believe they were generated automatically by GitHub.

They are, but you have the option to change them when you click "Commit".