BUG: fix groupby crash on duplicated columns (GH7511)
Fix creation of database indexes (GH8083)

Tests fix

Fixes

Remove _executefunc

Fixed tests to work with Python3

Fix merge

Index test simplify

Test fix
Merge pull request #8083 from artemyk/to_sql_create_indexes

BUG: When creating table, db indexes should be created from DataFrame indexes
API: raise on setops for + and - for Indexes (GH8226)
Merge pull request #8227 from jreback/index_api

API: raise on setops for + and - for Indexes (GH8226)
DOC: created advanced indexing section
DOC: add internals section (GH4082)