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* enum: Add Enum.value property

* simplify

* address review

* minor cleanup: fixing or silencing flake8 errors

* ci: lock CMake to non-Universal version

* Update .github/workflows/ci.yml
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Add argument names to enum_ methods

* Add test_enum::test_docstring_signatures

* style: use Black everywhere

* style: minor touchup from review

This changes enum reprs to look like `<Enum.name: value>` similarly to
the Python enum module.

This keeps the str of enums as `Enum.name`, like the Python enum module.

* fix: Avoid conversion to `int_` rhs argument of enum eq/ne

* test: compare unscoped enum with strings

* suppress comparison to None warning

* test unscoped enum arithmetic and comparision with unsupported type

This commit addresses an inefficiency in how enums are created in
pybind11. Most of the enum_<> implementation is completely generic --
however, being a template class, it ended up instantiating vast amounts
of essentially identical code in larger projects with many enums.

This commit introduces a generic non-templated helper class that is
compatible with any kind of enumeration. enum_ then becomes a thin
wrapper around this new class.

The new enum_<> API is designed to be 100% compatible with the old one.

* check for already existing enum value added; added test

* added enum value name to exception message

* test for defining enum with multiple identical names moved to test_enum.cpp/py

The property returns the enum_ value as a string.
For example:

>>> import module
>>> module.enum.VALUE
enum.VALUE
>>> str(module.enum.VALUE)
'enum.VALUE'
>>> module.enum.VALUE.name
'VALUE'

This is actually the equivalent of Boost.Python "name" property.

This udpates all the remaining tests to the new test suite code and
comment styles started in #898.  For the most part, the test coverage
here is unchanged, with a few minor exceptions as noted below.

- test_constants_and_functions: this adds more overload tests with
  overloads with different number of arguments for more comprehensive
  overload_cast testing.  The test style conversion broke the overload
  tests under MSVC 2015, prompting the additional tests while looking
  for a workaround.

- test_eigen: this dropped the unused functions `get_cm_corners` and
  `get_cm_corners_const`--these same tests were duplicates of the same
  things provided (and used) via ReturnTester methods.

- test_opaque_types: this test had a hidden dependence on ExampleMandA
  which is now fixed by using the global UserType which suffices for the
  relevant test.

- test_methods_and_attributes: this required some additions to UserType
  to make it usable as a replacement for th...

The added flag enables non-buffered console output when using Ninja

Enumerations on Python 2.7 were not always implicitly converted to
integers (depending on the target size). This patch adds a __long__
conversion function (only enabled on 2.7) which fixes this issue.

The attached test case fails without this patch.

A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs

* Allow enums to be ordered
* Support binary operators

There are more enum tests than 'constants and functions'.