Django-x509

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Changes
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- Models: updated ``max_length`` of ``common_name`` (``Ca`` and ``Cert``)
to from ``63`` to ``64``, following conventions about maximum length of
common names and hostnames

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Features
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- Added swappable models, improved extensibility
- Improved documentation on `how to extend django-x509 <https://github.com/openwisp/django-x509#extending-django-x509>`_

Changes
~~~~~~~

- **Breaking change**: systems using django-x509 as a library must set ``DJANGO_X509_CA_MODEL``
& ``DJANGO_X509_CERT_MODEL`` values in their settings.py when upgrading or an exception like the following one will be raised:

``django.core.exceptions.ImproperlyConfigured: Could not find django_x509.Ca!``
- Added support for django 3.1
- Added support for cryptography 3.0.0

Bugfixes
~~~~~~~~

N/A

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- Added possibility to renew CAs and certificates
- Updated dependency to support cryptography 2.9

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- Switched back to jsonfield

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- Ensured RFC5280 datetime standard
- Increased maximum length of x509 serial number to 48
- jsonfield2 version set to >=3.1.0,<4.0.0

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- Dropped support for python 2
- Added support for django 3.0