Use correct PEP 440 fallback version

[johnmhoran]

The current fallback version generates an incorrect result when git export-subst is applied to replace the placeholders; and pip and setuptools complain with this error message (when using fetchcode@0.3.0 to explore the process):

setuptools.extern.packaging.version.InvalidVersion: Invalid version: '9999.d0a3fa9-2023-12-19'

We should instead use this modified fallback version:

fallback_version = "9999+$Format:%h$"

See, e.g.,

• https://peps.python.org/pep-0440/
• https://packaging.python.org/en/latest/specifications/version-specifiers/#local-version-identifiers
• https://git-scm.com/docs/gitattributes#_creating_an_archive

and

• https://github.com/nexB/fetchcode/blob/v0.3.0/pyproject.toml#L8 (example of our current fallback version syntax -- this generated the InvalidVersion error noted above)

[tool.setuptools_scm]
# this is used populated when creating a git archive
# and when there is .git dir and/or there is no git installed
fallback_version = "9999.$Format:%h-%cs$"

• https://github.com/nexB/fetchcode/blob/v0.3.0/.gitattributes

# Ignore all Git auto CR/LF line endings conversions
* -text
pyproject.toml export-subst