Replace recursion with stack (fix #115)

[haydn-jones]

Changed _derive_smiles_from_fragment to use a stack rather than recursion as #115 is triggered by hitting the stack limit.

Test suite passed and to verify further I compared the encodings AND decodings of every smile in the test set csv's with and without this change (there were no differences).

Closes #115.

Edit: test suite passed up to the linting errors, which do not seem to be new.

[MarioKrenn6240]

Thanks for the PR. I have had a brief look at it and as i understand it looks good. Alston Lo (who actually wrote the entire base code of v2.x) will have a look at it too very soon.

in the meantime, we decided to remove the linter from the CI, could u do so aswell please in your PR? also, i think your example if a great additional case for the CI, could u add it to test_specific_cases.py?

We will announce this as v2.1.2 when its merged.

[haydn-jones]

Pulled the recent changes and added the test case. For my own peace of mind I also compared the decodings of the current and previous implementations for ~130 million molecules from ZINC20 and everything checked out.