fix #671: hide NoReturn from very old python 3.6

CHANGELOG.rst

@@ -1,3 +1,9 @@
+v6.4.2
+======
+
+* fix #671 : NoReturn is not avaliable in painfully dead python 3.6
+
+
 v6.4.1
 =======
 

src/setuptools_scm/__init__.py

@@ -4,8 +4,8 @@
 """
 import os
 import warnings
-from typing import NoReturn
 from typing import Optional
+from typing import TYPE_CHECKING
 
 from . import _types
 from ._entrypoints import _call_entrypoint_fn
@@ -27,6 +27,10 @@
 from .version import meta
 from .version import ScmVersion
 
+if TYPE_CHECKING:
+    from typing import NoReturn
+
+
 TEMPLATES = {
     ".py": """\
 # coding: utf-8
@@ -98,7 +102,7 @@ def _do_parse(config: Configuration) -> "ScmVersion|None":
     return version
 
 
-def _version_missing(config) -> NoReturn:
+def _version_missing(config) -> "NoReturn":
     raise LookupError(
         f"setuptools-scm was unable to detect version for {config.absolute_root}.\n\n"
         "Make sure you're either building from a fully intact git repository "