Remove contains and iter from PyMappingProtocol.

[sebpuetz]

The methods are not expected by CPython and are only explicitly
callable. To get iteration support, PyIterProtocol should be
implemented and to get support for x in mapping,
PySequenceProtocol's contains should be implemented.

#611

Thank you for contributing to pyo3!

Here are some things you should check for submitting your pull request:

• Run cargo fmt (This is checked by travis ci)
• Run cargo clippy and check there are no hard errors (There are a bunch of existing warnings; This is also checked by travis)
• If applicable, add an entry in the changelog.
• If applicable, add documentation to all new items and extend the guide.
  • Could be nice to eventually add a Rust implementation of a PyDict under examples.
• If applicable, add tests for all new or fixed functions

You might want to run tox (pip install tox) locally to check compatibility with all supported python versions. If you're using linux or mac you might find the Makefile helpful for testing.

[kngwyu]

I checked CPython codes, and found no reason we have these methods.

[kngwyu]

Thanks!
Looks CI fails because of rustfmt check.
Could you please run cargo fmt on your local machine?

[sebpuetz]

Sure, amended the commit!

Looks like I forgot to run that after removing the defs from the derive backend.

[kngwyu]

Thanks, again.