Add checker for relationship backref usage

[miketheman]

Related to #25

This pull request introduces a new checker to the flake8-sqlalchemy plugin that detects the use of backref in SQLAlchemy relationship() calls and recommends using back_populates instead. This change aims to encourage best practices in SQLAlchemy relationship definitions.

• Implements a new checker: Adds RelationshipBackrefChecker in flake8_sqlalchemy/checkers/relationship_backref_checker.py to scan for relationship() calls that use backref and suggest back_populates.
• Updates plugin architecture: Modifies flake8_sqlalchemy/checkers/__init__.py and flake8_sqlalchemy/plugin.py to include the new RelationshipBackrefChecker in the list of checkers and in the visitor's checker dictionary, respectively.
• Adds tests: Introduces tests in tests/checkers/test_relationship_backref_checker.py to verify that the checker accurately identifies relationship() calls with backref and does not raise false positives for those using back_populates.

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[miketheman]

Refactors line lengths in flake8_sqlalchemy/checkers/relationship_backref_checker.py to comply with the 88 character limit.

• Breaks long lines in docstrings for the run and is_relationship_with_backref methods to ensure compliance with the project's line length restriction.
• Maintains readability and code structure while adhering to the .flake8 configuration.

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[miketheman]

Adds documentation for the SQA300 check in the README.md file.

• Introduces a new section for SQA300 to guide users on using back_populates instead of backref in SQLAlchemy relationships.
• Provides examples of incorrect usage with backref and correct usage with back_populates, following the existing documentation structure for other checks.

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