Support Python 3.9 in developer tools

[jsoriano]

Python 3.9 is planned to be released on October 2020, on this version some long-time deprecated code will be removed and some old Python libraries will stop working (see this). Check how this will affect Beats development and prepare the code to work on this version.

Identified issues

• Nosetests was discontinued on 2015 and has been reported to stop working on Python 3.9. Migrate tests to pytest (continue with Use pytest instead of nosetests #16883).
• Pillow will stop working on 3.9, 8.0 release planned Release Pillow 8.0.0 on October 15, 2020 python-pillow/Pillow#4764 (removed Remove pillow from testing requirements.txt #20407).
• Elasticsearch python library Upgrade elasticsearch client library used in tests #20405.
• Review deprecation warnings and address them when possible (see DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' #19915).
• Consider making deprecation warnings to provoke errors (filterwarnings = error::DeprecationWarning in pytest.ini) Fix python deprecation warnings #20603.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)