Remove support for end-of-life Python 2.7

Python 2.7 is end of life. It is no longer receiving bug fixes,
including for security issues. Python 2.7 went EOL on 2020-01-01. For
additional details on support Python versions, see:

Supported: https://devguide.python.org/#status-of-python-branches
EOL: https://devguide.python.org/devcycle/#end-of-life-branches

Removing support for EOL Pythons will reduce testing and maintenance
resources while allowing the library to move towards a modern Python 3
style. Python 2.7 users can continue to use the previous version of
redis-py.

Was able to simplify the code:

- Removed redis._compat module
- Removed __future__ imports
- Removed object from class definition (all classes are new style)
- Removed long (Python 3 unified numeric types)
- Removed deprecated __nonzero__ method
- Use simpler Python 3 super() syntax
- Use unified OSError exception
- Use yield from syntax

Co-authored-by: Andy McCurdy <andy@andymccurdy.com>

This allows memoryview instances to be passed to Redis command args that
expect strings or bytes. The memoryview instance is sent directly to
the socket such that there are zero copies made of the underlying data
during command packing.

Fixes #1265
Fixes #1285

Pass encoding_errors setting to hiredis (>=1.0.0).

Fixes #1161

The Token class was needed when supporting Python 2.6. Now that we've
dropped support for 2.6, we don't need it anymore.

Fixes #1066

All input sent to Redis is coerced into bytes. This includes key names and
values. Prior to this change, redis-py made an effort to cooerce all input
into strings by calling str() (Python 3) or unicode() (Python 2). While this
works for a handful of types like ints, longs and floats, it fails for other
types like bools ('True' or 'False'), None ('None') and many user defined
types.

Starting with redis-py version 3.0, sending input of any other type is
considered an error an a DataError exception will be raised.

Fixes #471
Fixes #472
Fixes #321
Fixes #190

Remove workaround for handling unicode with older Pythons.

All supported Python versions support the with statement.

Fixes warning output:

DeprecationWarning: "charset" is deprecated. Use "encoding" instead