Un-pin setuptools_scm for Python 3.x

Blocking current setuptools releases has blocked building on *current*
Python environments in the name of compatibility with EOL's 2.x releases.

Co-authored-by: Edgar Ramírez Mondragón <16805946+edgarrmondragon@users.noreply.github.com>

pyproject.toml

@@ -3,7 +3,8 @@ requires = [
     "setuptools; python_version != '3.3'",
     "setuptools<40.0; python_version == '3.3'",
     "wheel",
-    "setuptools_scm<8.0"
+    "setuptools_scm; python_version >= '3.6'",
+    "setuptools_scm<8.0; python_version < '3.6'"
 ]
 build-backend = "setuptools.build_meta"