[WIP] Migrate away from legacy Python versions

[nocarryr]

Targeting modern Python (>=3.8) will allow for cleaner code, proper typing support (planned soon) and reduce technical debt (to name a few).

Any legacy users would still be able to install older versions of pyrtlsdr (automatically handled by setuptools python_requires in the metadata).

Not sure what the roadmap of this change should like, but I think it's a good time to start that discussion

[coveralls]

Pull Request Test Coverage Report for Build 7324288833

• 12 of 12 (100.0%) changed or added relevant lines in 5 files are covered.
• 25 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.3%) to 86.952%

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Files with Coverage Reduction	New Missed Lines	%
rtlsdr/rtlsdraio.py	8	87.3%
rtlsdr/librtlsdr.py	17	73.83%

Totals
Change from base Build 7324163658:	-2.3%
Covered Lines:	813
Relevant Lines:	935

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💛 - Coveralls