Modernize the repo a bit

[nikolaik]

• Fix markdown deprecation warnings
• Use pathlib instead of os module
• Use subprocess.run
• Format with black
• Add basic test
• Run tests on CI
• Update classifiers

[johanneswuerbach]

Thanks, this looks great 😍

[johanneswuerbach]

Confirmed the tests to be working on github actions https://github.com/johanneswuerbach/markdown-inline-mermaid/pull/2 🎉

[johanneswuerbach]

Released as 1.0.3 🚀

[nikolaik]

Nice, thanks for the quick review and release @johanneswuerbach !