Open Babel wrapper can't deal with gzipped files

[RMeli]

Describe the bug

python -m spyrmsd with Open Babel backend for I/O does not work with gzipped files.

Traceback (most recent call last):
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/site-packages/spyrmsd/__main__.py", line 33, in <module>
    ref = io.loadmol(args.reference)
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/site-packages/spyrmsd/io.py", line 66, in loadmol
    mol = load(fname)
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/site-packages/spyrmsd/optional/obabel.py", line 33, in load
    obmol = next(pybel.readfile(fmt, fname))
  File "/home/lina3015/.conda/envs/gsoc/lib/python3.6/site-packages/openbabel/pybel.py", line 159, in readfile
    raise ValueError("%s is not a recognised Open Babel format" % format)
ValueError: gz is not a recognised Open Babel format

To Reproduce

python -m spyrmsd reference.sdf.gz docking.sdf.gz

Expected behavior

python -m spyrmsd should work with compressed files since Open Babel can deal with those.

Environment

• OS: CentOS Linux 8
• Python: Python 3.6.13
• Package Manager [e.g. pip, conda]: conda

Additional Context

For file.ext.gz, the following return gz as extension:

spyrmsd/spyrmsd/utils.py

Line 25 in 00207e3

name, ext = os.path.splitext(fname)

File extensions are used as format string for pybel

and therefore lead to errors.

spyrmsd/spyrmsd/optional/obabel.py

Lines 31 to 33 in 00207e3

	fmt = utils.molformat(fname)
	obmol = next(pybel.readfile(fmt, fname))

The function

spyrmsd/spyrmsd/utils.py

Lines 30 to 32 in 00207e3

	def molformat(fname: str) -> str:
	"""
	Extract an OpenBabel-friendly format from file name.

needs to deal with this problem.