closes 360
from https://github.com/JiKeidan

…#358)

* regenerate pylintrc for newer standards.
* Remove outdated setattr usages.
* Remove outdated todo statements.
* Fix whitespace and line length linting.
* Update ruamel.yaml usage to avoid deprecation warnings.
* add default timeout to requets calls for registry swagger.
* Fixes #351
* Fixes #342
* Fix HTTPResponse.getheaders() deprecation warning with py2/py3 compatitbility
* Call Configuration() once in rest clients
* added explanation about safe yaml loader
* ensure latest flow version by sorting. Fixes #326
* set py>-1.11.0 to avoid false positive cve warning
* bump pytest>=7.2.0 to remove py dependency

* update clients to 1.27.0

* deprecate py.test in favour of newer pytest

* move docker requirement to extras

* linting fixes

* set latest python version to 3.12

simplify yaml dump to remove duplicate round trip via json
fix py2/3 compatibility on use of yield statements.
remove automated py2 testing from tox due to compatibility issues
move pytest version picker to setup.py to allow additional logic.
remove extraneous settings from setup.cfg
set different yaml parser rules for py2/3 to better handle compatibility
remove py2 limitation on development versions, greatly simplifying setup
update pylintrc for new standards
retire travisCI and coveralls from automated testing.
Set simple coverage report, exclude from commits.

update devnotes with py2 manual testing
deprecate removed models from nifi client docs

Prepare v0.21.0 with py2 semi-retirement

…368)

* Instruct pyup to ignore pytest in requirements txt
* instruct pytest to not report insecure http requests to localhost for docker testing
* Templates are deprecated in nifi-2x, remove from tests for newer versions
* add function to check registry version using newer aboutapi() call
* update utils.check_version to ignore any non-semver elements of a version string and use newer registry version api call if available
* Update registry client creation to handle nifi-2x structure
* Update Testing to treat NiFI-2.x as regression target
* Deprecate very old nifi-1x versions from regression testing
* Fix get processor test to use a processor available in all nifi versions, GenerateFlowfile
* Update readme about NiFi2.x support.
* Switch 'latest' dockerfile to use 'latest' nifi tags.