Add graph colouring

[AswinPKumar01]

Describe your change:

• [x] Add an algorithm: Implemented the Graph Coloring algorithm using backtracking. This algorithm assigns colors to vertices of a graph such that no two adjacent vertices share the same color.

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in lowercase with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.