PEP8 formatting

[Zhigal]

Sorry for a HUGE pr...

In this PR:

• PEP8 formatting applied:
  • docstring converted to """ from '''
  • emply lines added where needed
  • self replaced with cls for class methods
• proofreading
• mutable arguments for some methods were replaced with immutable
• unused imports removed
• None comparison replaces with is None check
• unnecessary parenthesis removed
• array initialization optimized for this cases:

arr = []
arr.append(...)

The logic is still the same, no changes here