Add a workaround for python-poetry/poetry#7702.

setup.py

@@ -1,13 +1,18 @@
 from setuptools import setup
-from setuptools_scm.git import parse as parse_git
 
 
 def doc_version():
+    try:
+        from setuptools_scm.git import parse as parse_git
         git = parse_git(".")
         if git.exact:
             return git.format_with("v{tag}")
         else:
             return "latest"
+    except ImportError:
+        # PEP 517 compliant build tools will never reach this code path.
+        # Poetry reaches this code path.
+        return ""
 
 
 setup(