Lower requirement to Python 3.8

[haja-fgabriel]

In our production environment, we use Python 3.8, and this library would really help us in our work. We can’t easily switch to a newer Python version.

Let me know if you have Python 3.9+ behavior apart from union type notations.

Changes done:

• removed Python 3.10 style of union types
• removed parenthesized context managers specific to Python 3.10

[haja-fgabriel]

Tested with the Jupyter notebook on Python 3.8.10 in a Codespace environment; after fixing the path to the test files, works smoothly.
@uhermjakob @berquist @thammegowda @joelthe1 it would be awesome if my modifications would make it into production :)