Add check for valid function postcondition syntax

[PraneethS42]

Proposed Changes

This pull request works on #1302. Adds to the checker contract_checker.py by adding logic to alert the user if a function docstring contains invalid syntax within one of its postconditions. This also leverages the pre-existing parse_assertions method from python-ta/contracts/__init__.py.

Adds an example file and appropriate changes in documentation, as well as tests inside the same test_contract_checker.py file, but within a different class.

...

Screenshots of your changes (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)	x
🐛 Bug fix (non-breaking change that fixes an issue)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📚 Documentation update (change that only updates documentation)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

(Complete each of the following items for your pull request. Indicate that you have completed an item by changing the [ ] into a [x] in the raw text, or by clicking on the checkbox in the rendered description on GitHub.)

Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
  • This is required for all bug fixes and new features.
• I have updated the project documentation, if applicable.
  • This is required for new features.
• I have updated the project Changelog (this is required for all changes).
• If this is my first contribution, I have added myself to the list of contributors.

After opening your pull request:

• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported by Coveralls.
• I have requested a review from a project maintainer.

Questions and Comments

(Include any questions or comments you have regarding your changes.)

[coveralls]

Pull Request Test Coverage Report for Build 22266478072

Details

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.984%

---

Totals
Change from base Build 22209214024:	0.02%
Covered Lines:	3450
Relevant Lines:	3834

---

💛 - Coveralls

[david-yz-liu]

Make sure to update this docstring as well

[david-yz-liu]

The string $return_value should be defined in a constant somewhere in the python_ta.contracts module; please look there and use the constant

[david-yz-liu]

rename cleaned_condition as well (in general prefer not reusing the same variable name across different contexts)

[PraneethS42]

Implemented, also decided to use the string replace() method to substitute the string as it felt more intuitive

[david-yz-liu]

Nice work, @PraneethS42!