Set setuptools_scm fallback_version to follow  PEP440
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
[tool.setuptools_scm]
local_scheme = "node-and-date"
-fallback_version = "unknown"
+fallback_version = "999.999.999+unknown"
[tool.coverage.run]
omit = ["*/tests/*", "*pygmt/__init__.py"]
