Drop Python2 support

[mohabusama]

Python2 EOL is coming soon, and we should support Python3-only and move on!

[ubersan]

This might make it possible to remove the future dependency and the calls to standard_library.install_aliases() (

opentracing-utils/opentracing_utils/common.py

Line 3 in 4fc8e51

standard_library.install_aliases() # noqa

)

This call seems to make use of the deprecated imp module. This is annoying as our tests will print a warning

.../python3.9/site-packages/future/standard_library/__init__.py:65
 .../lib/python3.9/site-packages/future/standard_library/__init__.py:65: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

-- Docs: https://docs.pytest.org/en/stable/warnings.html

There is an open upstream issue (PythonCharmers/python-future#246), but it's open for a few years now.