Drop support for Python < 3.8 and remove CircleCI (#177)

Initial plan

Drop support for Python versions < 3.8 and remove CircleCI build


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Fix rst-lint command - remove deprecated –encoding flag

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Update CONTRIBUTING.md to match new rst-lint command

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Remove Python < 3.8 version checks from C code and update why_ciso8601.md

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Improve comment consistency - clarify Python 3.6 vs 3.8 requirements

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Drop support for Python < 3.8 and remove CircleCI

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Fix clang-format issues in module.c

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Remove redundant “Since we require Python 3.8+” from comments

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Remove redundant comment about Python 3 error format

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Remove unused PY_VERSION_AT_LEAST_36 macro from timezone.c

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Simplify why_ciso8601.md text about older Python versions

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Address PR comments from thomasst and movermeyer


Remove unused PY_VERSION_AT_LEAST_38 macro (thomasst)
Simplify SUPPORTS_37_TIMEZONE_API macro using suggested syntax (thomasst)
Add section for Python < 3.8 support pointing to v2.x (movermeyer)
Update flowchart to include v2.x option for older Python versions

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