TYP: annotation of __init__ return type (PEP 484) (pandas/core/computation)

[EkaterinaKuzkina]

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According to documentation pandas uses PEP484 for type hints.
PEP484 states that __init__ function should have -> None return type annotation.
In this PR I added return type annotation to __init__ functions in pandas/core/computation module

[jreback]

thanks @EkaterinaKuzkina

[twoertwein]

While I prefer having the explicit None annotation, I think @simonjayhawkins (and others) did not like the idea of adding None, I think because it is not needed, __init__ can never return anything else.