Handle joins in arithmetic expressions

[henadzit]

Description

• Returns joins from arithmetic expressions (CombinedExpression) which allows to have queries like Author.annotate(rating=Avg(F("books__rating") + 1))
• Moves TestQueryReuse tests to a separate file. I found it to be quite inconvenient to have them in test_queryset, I added a few unrelated tests to TestQueryReuse by mistake.

Motivation and Context

This should fix #996.

How Has This Been Tested?

• Added tests

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[henadzit]

Moved to test_queryset_reuse.py

[coveralls]

Pull Request Test Coverage Report for Build 11766677963

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.162%

---

Totals
Change from base Build 11764062074:	0.0%
Covered Lines:	6084
Relevant Lines:	6709

---

💛 - Coveralls

[abondar]

@henadzit Thank you!

If you are by any chance willing to be part of tortoise and help me maintain it - I would be happy to invite you.
If so - please contact me at [email protected]

P.S. Do I understand correctly that you are russian-speaking? :)