fix: guard rdkit-dependent constants in atoms.py

[mulatta]

Reference Issues/PRs

None found. This is a previously unreported bug.

What does this implement/fix? Explain your changes

graphein/molecule/atoms.py defines module-level constants using Chem.rdchem.* enums (e.g. ALLOWED_HYBRIDIZATIONS, ALL_BOND_TYPES), but the existing try/except block only guards the import rdkit.
Chem statement. When rdkit is not installed, these constants raise NameError, making import graphein.molecule fail entirely.

This contradicts the design intent visible in other molecule submodules (graphs.py, utils.py) where
rdkit is treated as an optional dependency with a flag-based guard pattern.

Fix: Add a _HAS_RDKIT flag to the existing try/except and wrap rdkit-dependent constants in if _HAS_RDKIT: / else: block with empty fallbacks. Pure Python constants (BASE_ATOMS, EXTENDED_ATOMS,
ALLOWED_DEGREES, etc.) are unchanged.

What testing did you do to verify the changes in this PR?

• Verified import graphein.molecule and import graphein.molecule.atoms succeed in a Python environment without rdkit installed
• Ran black --check and isort --check-only on the changed file — both pass

Pull Request Checklist

• Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./graphein/tests/* directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/protein/test_graphs.py)
• Checked for style issues by running black . and isort .

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mulatta]

it seems pre-commit failure is not associated with this diff.