feature: support compilers that use std::experimental::filesystem  (#3840)

* feature: support compilers that use std::experimental::filesystem such as gcc7

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* only use this feature if cpp17

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* more specific namespace alias + style

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* move to pybind11 namespace

* no namespace alias

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: Lonnie Souder II <lonnie.souder@g6labs.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>