git: fix upload-pack content-type with py3

It is checked earlier that git_command is one of two string constants, and with
py3, things are much simpler and we don't have to consider string coersion.

Reported and debugged by Valentin.

isort: upgrade to 5.1.2

The changes to non-top-level imports seem nice.

py3: remove safe_unicode in places where it no longer is needed because all strings (except bytes) already *are* unicode strings

(The remaining safe_unicode calls are still needed and can't just be removed,
generally because we in these cases still have to convert from bytes to unicode
strings.)

pygrack: address webob AcceptValidHeader DeprecationWarning

Got warnings like:
kallithea/tests/other/test_vcs_operations.py::TestVCSOperations::test_yada_yada
.../site-packages/webob/acceptparse.py:649: DeprecationWarning: The behavior of AcceptValidHeader.__contains__ is currently being maintained for backward compatibility, but it will change in the future to better conform to the RFC.
DeprecationWarning,

There *must* be a better alternative to pygrack ... for now, let's keep it alive.

middleware: simplify pygrack wrapping - there is no need for any extras

For symmetry, do the same in simplehg where it is completely unused.

codingstyle: trivial whitespace fixes

Reported by flake8.

git: include an LF at the end of the service advertisement (Fixes #230)

This fixes hg-git/Dulwich and possibly other conservative Git clients,
which do not ignore the absence of the LF.

The original comment was a guess based on reverse engineering the protocol
not specified in the documentation yet at that moment. Now that the
documentation exists and states this explicitly, just do as it says.

git: include an LF at the end of the service advertisement (Fixes #230)

This fixes hg-git/Dulwich and possibly other conservative Git clients,
which do not ignore the absence of the LF.

The original comment was a guess based on reverse engineering the protocol
not specified in the documentation yet at that moment. Now that the
documentation exists and states this explicitly, just do as it says.

git: make sure repository path and name are Unicode strings

This is needed for supporting latest Dulwich version, but it also works with
out supported version.

pygrack: refactor _get_fixedpath

This function strips the repository name and any slashes from the
URL path, leaving the protocol command, like info/refs or
git-{receive,upload}-pack.

Using .split() and .strip() for this purpose isn't entirely reliable
and is entirely unreadable, so it's better to write it out, even if
it's a bit verbose.

Also, error out in the unlikely case the path doesn't start with
a slash and the repository name; that shouldn't happen normally.

pygrack: add missing author and copyright information

This module is based on code from git_http_backend.py,
adapted and integrated by Marcin Kuźmiński.

The protocol request handling comes from Daniel, the
rest of the code seems to come from Marcin.

cleanup: consistently use 'except ... as ...:'

Use the Python 2.6+ syntax instead of the old confusing 'except ..., ...'
syntax.

git: run external commands as list of strings so we really get correct quoting (Issue #135)

a6dfd14d4b20 from
https://bitbucket.org/conservancy/kallithea/pull-request/17/add-quotes-to-repo-urls-for-git-backend
fixed that issue but did not make it "safe". The vcs git backend still used
command strings but tried to quote them correctly ... but that approach is
almost impossible to get right.

Instead, pass a string list all the way to the subprocess module and let it do
the quoting. This also makes some of the code more simple.

Second step in two-part process to rename directories.
This is the actual directory rename.