Keyring

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* 357: Once again allow all positive, non-zero priority
keyrings to participate.

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* 323: Fix race condition in delete_password on Windows.
* 352: All suitable backends (priority 1 and greater) are
allowed to participate.

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* 350: Added new API for ``get_credentials``, for backends
that can resolve both a username and password for a service.

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* 340: Add the Null keyring, disabled by default.
* 340: Added ``--disable`` option to command-line
interface.
* 340: Now honor a ``PYTHON_KEYRING_BACKEND``
environment variable to select a backend. Environments
may set to ``keyring.backends.null.Keyring`` to disable
keyring.

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Removed deprecated ``keyring.util.escape`` module.

Fixed warning about using deprecated Abstract Base Classes
from collections module.

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Removed ``getpassbackend`` module and alias in
``keyring.get_pass_get_password``. Instead, just use::

keyring.get_password(getpass.getuser(), 'Python')