Add fallback version for setuptools_scm

This will work even from a git archive or when git is not installed.

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

Diff for: .gitattributes

@@ -1,2 +1,3 @@
 # Ignore all Git auto CR/LF line endings conversions
 * -text
+pyproject.toml export-subst

Diff for: pyproject.toml

@@ -3,6 +3,7 @@ requires = ["setuptools >= 50", "wheel", "setuptools_scm[toml] >= 4"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]
+fallback_version = "v9999.$Format:%h-%cs$"
 
 [tool.pytest.ini_options]
 norecursedirs = [