Requests-kerberos

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- Added support for proxy authentication with ``HTTP`` endpoints.
- Support for proxying ``HTTPS`` endpoints is not available due to limitations
of the underlying requests/urllib3 library.
- Fixed up stray bytes to str conversion.

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- Change Kerberos dependencies to pyspnego_ to modernise the underlying
Kerberos library that is used.
- Removed the ``wrap_winrm`` and ``unwrap_winrm`` functions
- Dropped support for Python 2 and raised minimum Python version to 3.6.
- Renamed the ``context`` attribute to ``_context`` to indicate it's meant for
internal use only.
- Fix Negotiate header regex pattern to avoid DoS affected patterns

.. _pyspnego: https://github.com/jborean93/pyspnego

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- Add support for channel binding tokens (assumes pykerberos support >= 1.2.1)
- Add support for kerberos message encryption (assumes pykerberos support >= 1.2.1)
- Misc CI/test fixes

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- Switch dependency on Windows from kerberos-sspi/pywin32 to WinKerberos.
This brings Custom Principal support to Windows users.

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- Make it possible to receive errors without having their contents and headers
stripped.
- Resolve a bug caused by passing the ``principal`` keyword argument to
kerberos-sspi on Windows.

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- Support for principal, hostname, and realm override.

- Added support for mutual auth.