Fix installation via "setup.py install"

Author: mnaberez

setup.py

@@ -22,11 +22,13 @@
 elif (3, 0) < py_version < (3, 4):
     raise RuntimeError('On Python 3, Supervisor requires Python 3.4 or later')
 
-# setuptools is required as a runtime dependency only on
-# Python < 3.8.  See the comments in supervisor/compat.py.
-requires = [
-    "setuptools; python_version < '3.8'",
-]
+# setuptools is required as a runtime dependency only on Python < 3.8.
+# See the comments in supervisor/compat.py.  An environment marker 
+# like "setuptools; python_version < '3.8'" is not used here because
+# it breaks installation via "python setup.py install".
+requires = []
+if py_version < (3, 8):
+    requires.append("setuptools")
 
 tests_require = []
 testing_extras = tests_require + [