add private method to enum to support replacing global constants with Enum members:
- search for candidate constants via supplied filter
- create new enum class and members
- insert enum class and replace constants with members via supplied module name
- replace __reduce_ex__ with function that returns member name, so previous Python versions can unpickle
modify IntEnum classes to use new method

Claudiu Popa.

The print_diffs() function was not defined, using the assertEqual() is more
reliable.

Close issue20534: test_enum now tests all supported pickle protocols (2 - HIGHEST_PROTOCOL, inclusive).

Also slight code reorg for PEP 8 guidelines.

Before this change only the methods were _single_underscored_; now
the attributes are as well.

The docs also clarify the 'Interesting Example' duplicate-free enum is for
demonstration purposes.

Missing files added.
News entry added.