Made it compatible to python3.7

Due to https://www.python.org/dev/peps/pep-0479/ stopIteration
will not be caught anymore and needs to be use return instead.
updated with relevant changes.

Change-Id: If25b6ccecd46fed0225a230096041996fb91c0c5
closes-bug: #1837778

glare/common/utils.py

@@ -542,7 +542,7 @@ class BlobIterator(object):
             bytes_left -= len(data)
             i += 1
             yield data
-        raise StopIteration()
+        return
 
 
 def validate_status_transition(af, from_status, to_status):

glare/tests/unit/test_utils.py

@@ -227,7 +227,7 @@ class TestReaders(base.BaseTestCase):
             yield chunk
             iteration += 1
             if iteration >= max_iterations:
-                raise StopIteration()
+                return
 
     def _test_reader_chunked(self, chunk_size, read_size, max_iterations=5):
         generator = self._create_generator(chunk_size, max_iterations)

tox.ini

@@ -8,7 +8,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          LANGUAGE=en_US
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 commands =
            /bin/rm -f .testrepository/times.dbm
@@ -24,8 +24,8 @@ commands = oslo_debug_helper {posargs}
 basepython = python2.7
 commands = oslo_debug_helper {posargs}
 
-[testenv:debug-py35]
-basepython = python3.5
+[testenv:debug-py37]
+basepython = python3.7
 commands = oslo_debug_helper {posargs}
 
 [testenv:pep8]