Since we decided to support only py3.7+ (and possibly 3.6) this PR aims to clean the code base from python2 old-style and compatibility constructs, including:

• old-style unicode literals (u'', u"")
• encoding declarations (comments # -*- coding: UTF-8 -*- at the top of a file)
• explicit class inheritance from object
• inheritance of the features already included in python 3.6+: unicode_literals, absolute_import, division, print_function (i.e. constructs in form from __future__ import "<feature>")
• allure_commons._compat.format_exception_only function and allure_commons._compat module altogether (traceback.format_exception_only is used instead)
• six compatability library:
  • Branches depending on six.PY2 was removed
  • six.with_metaclass replaced with metaclass= in object class declarations
  • six.text_type is always str in py3
  • six.string_types is always str in py3
• enum32 dependency of allure-python-commons: backport of enums for py3.4
• py2-related unicode-encoded json file writing algorithm in allure_commons.logger.AllureFileLogger._report_item
• allure_commons.utils.escape_non_unicode_symbols function (returns an argument itself in python3)
• allure_commons.getargspec function (inspect.getfullargspec is used in py3 instead)

Also made some refactorings and small changes:

• Replace %-formatting and str.format with string interpolation (f-strings) where is improves readability
• Add supported python version classifiers in setup.py files
• Update python version to 3.11 in linting step of the build workflow
• Add py3.11 to build&test matrix of the build workflow
• Update checkout to v3 and setup-python to v4 in the release workflow
• Factor out link formatting in allure_pytest.helper.AllureTestHelper.decorate_as_link and allure_pytest.listener.AllureListener.add_link to allure_pytest.utils.format_allure_link (following DRY)
• Fix allure_test/test/acceptance/fixture/yield_fixture_test.py::test_yield_fixture test to actually check a yield-fixture (not a usual one) - Fixes Allure-pytest: Yield_fixture test is invalid #551
• Refactor a link creation in allure_robotframework.listener.utils, make parse_link a module-level function (it has no dependencies on local variables)
• Remove dependency on dict keys ordering in py3.6 since it's an implementation detail of the CPython 3.6. Insertion order preservation became a part of the spec in py3.7. See here and here.