Optimize classes by adding `__slots__`

[GLEF1X]

Benchmarks show that adding __slots__ to the classes reduce memory usage by 4-6%.

Without __slots__:

With it:

[lgtm-com[bot]]

This pull request fixes 17 alerts when merging e6d24ebecb57d08addbd94009398f20d1df3ab9f into fcb9a93d12a15b37a95e485f861a72025b571180 - view on LGTM.com

fixed alerts:

• 16 for Unused import

• 1 for Module is imported with 'import' and 'import from'

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