Refactoring of `src/*.py` files

[JustKappaMan]

Hi @ynsta

I didn't change any logic and didn't break back compatibility with earlier Python versions.

Just fixed some stuff like:

• if x == None: -> if x is None:
• if len(lst) > 0: -> if lst:
• if not s or s == '': -> if not s:
• Formatting
• Typos

If you're interested in such improvements -- merge this PR and I'll work on the rest of the code in the project :slightly_smiling_face:

[JustKappaMan]

@ynsta any feedback? :slightly_smiling_face: