Switch to stestr, declare Python 3.7 compatibility

Since [1] the project supports Python 3.7. It is also a good idea
to move away from ostestr to stestr, as all OpenStack projects did
already.

[1] - https://review.opendev.org/715634

Change-Id: I68bb89e441cba373f3c781ec0f4418c3969c8f85

setup.cfg

@@ -12,9 +12,8 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
-    Programming Language :: Python :: 2.6
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.7
 
 [entry_points]
 console_scripts =

test-requirements.txt

@@ -1,5 +1,5 @@
 flake8
 sphinx>=1.6.1
-os-testr>=0.8.0 # Apache-2.0
+stestr>=1.0.0 # Apache-2.0
 mock>=2.0.0 # BSD
 testtools>=1.4.0 # MIT

tox.ini

@@ -15,7 +15,7 @@ deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = rm
 commands =
     rm -f .testrepository/times.dbm
-    ostestr {posargs}
+    stestr run {posargs}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY