Use ruff for linting

[aweakley]

I've had a crack at this. @ColeDCrawford please could you have a look at 17f7960 in particular? I think it's ok, but I might have missed something.

I'm left with these ruff errors, and to fix them I think we need to decide what should be included in __all__

edtf/__init__.py:2:5: F401 `edtf.convert.dt_to_struct_time` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:3:5: F401 `edtf.convert.jd_to_struct_time` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:4:5: F401 `edtf.convert.old_specs_to_new_specs_expression` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:5:5: F401 `edtf.convert.struct_time_to_date` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:6:5: F401 `edtf.convert.struct_time_to_datetime` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:7:5: F401 `edtf.convert.struct_time_to_jd` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:8:5: F401 `edtf.convert.trim_struct_time` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:10:26: F401 `edtf.natlang.text_to_edtf` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:11:33: F401 `edtf.parser.grammar.parse_edtf` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/__init__.py:12:1: F403 `from edtf.parser.parser_classes import *` used; unable to detect undefined names
edtf/fields.py:92:20: S301 `pickle` and modules that wrap it can be unsafe when used to deserialize untrusted data, possible security issue
edtf/natlang/__init__.py:1:17: F401 `.en.text_to_edtf` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/parser/__init__.py:1:33: F401 `edtf.parser.grammar.parse_edtf` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
edtf/parser/__init__.py:2:1: F403 `from edtf.parser.parser_classes import *` used; unable to detect undefined names
Found 14 errors.

[ColeDCrawford]

Have a PR to that branch for review: #51

[aweakley]

Thank you, I've merged it now.