Use compat fspath instead of os.fspath

When 7ba35c4 was cherry-picked it introduced the
usage of os.fspath which is not supported on
Python <3.6

CHANGES.rst

@@ -1,5 +1,12 @@
 .. currentmodule:: flask
 
+Version 1.1.4
+-------------
+
+Unreleased
+
+-   Update ``static_folder`` to use ``_compat.fspath`` instead of
+    ``os.fspath`` to continue supporting Python <3.6 :issue:`4050`
 
 
 Version 1.1.3

src/flask/__init__.py

@@ -57,4 +57,4 @@ from .signals import template_rendered
 from .templating import render_template
 from .templating import render_template_string
 
-__version__ = "1.1.3"
+__version__ = "1.1.4.dev0"

src/flask/helpers.py

@@ -1001,7 +1001,7 @@ class _PackageBoundObject(object):
     @static_folder.setter
     def static_folder(self, value):
         if value is not None:
-            value = os.fspath(value).rstrip(r"\/")
+            value = fspath(value).rstrip(r"\/")
         self._static_folder = value
 
     @property

tests/test_basic.py

@@ -1425,7 +1425,6 @@ def test_static_url_empty_path_default(app):
     rv.close()
 
 
-@pytest.mark.skipif(sys.version_info < (3, 6), reason="requires Python >= 3.6")
 def test_static_folder_with_pathlib_path(app):
     from pathlib import Path