Netdiff

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- [deps] Updated dependencies and dependency monitoring badge
- [change] Added compatibility for latest OLSRd1 jsoninfo format

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- **Backward incompatible change for OpenVPN Parser**:
switched to use common name as netjson ID to avoid duplicate nodes when
multiple nodes are behind the same public IP, for more information
see `Specialized features: OpenVPN <https://github.com/openwisp/netdiff#openvpn>`_

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- Fix: fixed parsing issue in OpenVPN which caused some node and links to
be missed if the OpenVPN nodes had same IP but different ports
- Feature: detect changes in nodes and links
- Change: Added/remove/changed nodes/links are now sorted
- Change: Unspecified fields like node's label and link's ``cost_text`` are now always
shown as empty string if they are not specified
- Change: Parse ``cost_text`` field from links
- **Backward incompatible change**: the output of ``diff`` in this release differs
slightly from the previous versions.
Applications using previous netdiff versions will likely need minor adjustments
to their code
- Documentation Improvements
- Added support for openwisp-utils~=0.5.0

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- Dropped support for python 2

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- Added ``DiGraph`` support for NetJsonParser
- Updated networkx version
- [openvpn] Ignore UNDEF nodes

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- `2283d6d <https://github.com/ninuxorg/netdiff/commit/2283d6d>`_:
[openvpn] Convert node properties to integer
- `6b72dab <https://github.com/ninuxorg/netdiff/commit/6b72dab>`_:
[openvpn] Avoid failing if data is empty
- `56 <https://github.com/ninuxorg/netdiff/pull/56>`_:
Updated networkx to 2.0