Remove `pytest` dependency in `pyproject.toml`

[MarcSerraPeralta]

pytest is listed as a dependency in pyproject.toml, but from a quick search in the whole repo, I see that it is only used in the test suit, meaning that it is not a required dependency for ldpc.

I would propose removing pytest from the dependncies and adding this line in pyproject.toml:

[project.optional-dependencies]
dev = ["pytest"]

With this change, pytest is not installed when doing pip install ldpc but one can install it for development purposes using pip install ldpc[dev].