Allow for processing of ForwardRefs

Features

• Introduce processing of ForwardRef in relations.
  Now you can create self-referencing models - both ForeignKey and ManyToMany relations.
  ForwardRef can be used both for to and through Models.
• Introduce the possibility to perform two same relation joins in one query, so to process complex relations like:

      B = X = Y
    //
   A 
    \
      C = X = Y <= before you could link from X to Y only once in one query
                   unless two different relation were used 
                   (two relation fields with different names)
  

• Introduce the paginate method that allows to limit/offset by page and page_size.
  Available for QuerySet and QuerysetProxy.

Other

• Refactoring and performance optimization in queries and joins.
• Add python 3.9 to tests and pypi setup.
• Update API docs and docs -> i.e. split of queries documentation.