Our user was experiencing issue [1] when using a git repository on NTFS mount
running on Linux. The current check checks if we are running on Windows, but
it should really check if we are on NTFS. And since checking fs type is not
that trivial and not efficient, it is simpler and better to just always apply
NTFS-specific logic, since it works on other filesystems as well.

[1] https://github.com/iterative/dvc/issues/1880#issuecomment-483253764

This should workaround possible permission issues.

Related to https://github.com/gitpython-developers/GitPython/issues/353

For some reason, it gets bytes where it did expect a stream ... .
Probably I should have figured out where this was input, instead
of fixing it the brutal way

autopep8 -v -j 8 --max-line-length 120 --in-place --recursive

It's a nice way of saying that there is still one failing, consistently.

Bytes should always be returned from the streams, so the tests
should be checking against byte strings instead of text strings.

This also fixes the `sha_iter` as it relied on the Python 2
`iterkeys` which has been renamed to `keys` in Python 3.

There were a few things which were being reused consistently for
compatibility purposes, such as the `buffer`/`memoryview` functions
as well as the `izip` method which needed to be aliased for
Python 3.  The `buffer` function was taken from `smmap` [1] and
reworked slightly to handle the optional third parameter.

This also adds a compatibility file dedicated entirely to encoding
issues, which seem to be the biggest problem.  The main functions
were taken in part from the Django project [2] and rewritten
slightly because our needs are a bit more narrow.

A constants file has been added to consistently handle the
constants which are required for the gitdb project in the core
and the tests.  This is part of a greater plan to reorganize
the `util.py` file included in this project.

This points the async extension back at the original repository
and points it to the latest commit.

[1]: https://github.com/Byron/smmap/blob/1af4b42a2354acbb53c7956d647655922658fd80/smmap/util.py#L20-L26
[2]: https://github.com/django/django/blob/b8d255071ead897cf68120cd2fae7c91326ca2cc/django/utils/encoding.py