Remove compatibility shims for entry points.

[jaraco]

• Remove SelectableGroups
• Remove DeprecatedList
• Remove compatibility for EntryPoints.__getitem__ by index.
• Remove support for cast of iterable of entry points to dict. Ref #97.
• Update changelog

[woutdenolf]

Starts to be rather cumbersome to support python>=3.7

import sys

try:
    from importlib.metadata import entry_points
except ImportError:
    from importlib_metadata import entry_points

if sys.version_info >= (3,10):
    _entry_points = entry_points(group=namespace)
else:
    try:
        _entry_points = entry_points().get(namespace, [])
    except AttributeError:
        _entry_points = entry_points().select(group=namespace)

[jaraco]

That's one way to do it. Other options include:

• Rely on importlib_metadata >= 3.6 for Python < 3.10. As a general rule, you should adopt importlib_metadata any time there's behavior from a future Python version but your application supports older Python versions.
• Use backports.entry_points_selectable (provides forward compatibility to Python 2.7+).
• Bypass entry_points altogether and re-implement its behavior using more fundamental interfaces (flake8 took this approach; I don't recommend it).

[woutdenolf]

Thanks for the feedback!

Rely on importlib_metadata >= 3.6 for Python < 3.10

Many projects just need importlib.metadata so they have importlib-metadata; python_version < '3.8'. Once python 3.7 gets dropped it should become simpler.

Python 3.10 still supports entry_points().select(group=namespace) so using entry_points(group=namespace) is not mandatory.