Drop support for EOL Python <= 2.6 & 3.0-3.2, add 3.5 & 3.6

Github -> GitHub

Remove dead code from __init__.py

I am investigating a migration to Python 3, and to facilitate this we are using the -3 flag as decribed here:
https://docs.python.org/3/howto/pyporting.html#prevent-compatibility-regressions . When using this flag I encountered
some issues inside of mock itself.

Python 3 now requires you to implement __hash__ if you implement __eq__. See https://docs.python.org/3.6/reference/datamodel.html#object.%5F%5Fhash%5F%5F .

```python
{mock.ANY}  # Fine in Python 2, Throws in Python 3
```

This PR explicitly sets the `__hash__` method on these objects as `None` to ensure the behavior is consistent in Python 3 as well as Python 2.

Sem-Ver: feature

Issue #26323: Add assert_called() and assert_called_once() methods to
unittest.mock.Mock.

Patch contributed by Robert Zimmerman.

original values after patching.

Patch contributed by Sean McCully.

boolean result instead of raising an exception.

Patch by A Kaptur.

Initial patch by Aaron Hill.

Pip 8 no longer supports 3.2, so we can't sensibly support it going
forward - but folk can use an older Mock if they need 3.2 support.

Fix typo in docs/index.txt

.travis.jml -> .travis.yml

We support 2.6 again, but 2.5 etc are not supported.

Rather than hand-checking, let setuptools do the checking. This avoids
issues with distutils being unable to parse some setuptools release
versions, which could bite us in future.

Patch from https://github.com/testing-cabal/mock/issues/215

This reverts commit e9db0161.

Issue #288 - mock_open.return_value.<stuff here> was a widely used pattern.

could in Python 3.3.

Patch from Nicola Palumbo and Laurent De Buyst.

Replace set comprehensions with calls to set() and ensure string
formatting has positional format specifiers.

Co-Authored-By: Robert Collins <rbtcollins@hp.com>