Fix issue #1318: Type (typehint) error for db.relationship

[cainmagi]

Fix the typehint inconsistence of db.relationship(...).

Rebased on the branch 3.1.x.

This work is continued from #1319

• fixes Type (typehint) error for db.relationship #1318
• Fix an issue when tox p fails because mypy is forbidden
  • This issue may be caused by upgrade of tox>=4. My tox version is 4.12.0.
  • See details here: https://stackoverflow.com/a/47716994/8266012

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
  • Only bug fixing. No need to do this.
• Add or update relevant docs, in the docs folder and in code.
  • Only bug fixing. No need to do this.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
  • Only bug fixing. No need to add any change logs for users.
• Add .. versionchanged:: entries in any relevant code docs.
  • Only bug fixing. No need to do this.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[jdimmerman]

Hey @davidism! Any thoughts on this PR?