Respect PydanticMeta::backward_relations

[Hummer12007]

Description

Implemented backward_relations for PydanticMeta: if it is set to False, fields of backward relationships will not be added to the Pydantic model.

Motivation and Context

Specifying backward_relations has no effect with current versions of tortoise, even though it is mentioned in the docs.

How Has This Been Tested?

Ran make test in the testing environment, will later add tests covering this feature.

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 read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 1872

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 96.813%

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Totals
Change from base Build 1868:	-0.01%
Covered Lines:	4221
Relevant Lines:	4312

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 432743649

• 15 of 15 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 96.355%

---

Totals
Change from base Build 431938944:	0.007%
Covered Lines:	4288
Relevant Lines:	4398

---

💛 - Coveralls

[lqmanh]

I hope this can be merged soon. It will solve my issue #510.

[lqmanh]

@grigi, @long2ice, @abondar
Please consider reviewing this PR, we really need this feature.

Thanks in advance!

[abondar]

Looks good, but also need documentation changes

And please write down changes to CHANGELOG
Also you could add yourself to contributors file

[abondar]

Ran make test in the testing environment, will later add tests covering this feature.

Also implementing tests would be nice

[Hummer12007]

Thanks!
I'll give it a shot during the weekend.

[Hummer12007]

Implemented tests, updated CHANGELOG, added myself to CONTRIBUTORS.

The class is mostly undocumented so I left docs as is.

[Hummer12007]

@abondar I think I've done everything.

[Hummer12007]

Thanks!

[abondar]

@Hummer12007 released as 0.16.19