Configure git-archive to export information for setuptools_scm.

This requires git>=2.32.0.

.git_archival.txt

@@ -0,0 +1,4 @@
+node: $Format:%H$
+node-date: $Format:%cI$
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
+ref-names: $Format:%D$

.gitattributes

@@ -1 +1,2 @@
+.git_archival.txt export-subst
 /amaranth/vendor/* -linguist-vendored

setup.py

@@ -4,9 +4,7 @@ from setuptools_scm.git import parse as parse_git
 
 def doc_version():
     git = parse_git(".")
-    if not git:
+    if git.exact:
-        return ""
-    elif git.exact:
         return git.format_with("v{tag}")
     else:
         return "latest"