Migrate from testr to stestr

* Replace .testr.conf by .stestr.conf for migration and update
  .gitignore and test-requirements.txt files accordingly
* Use py3 as the default runtime for tox and 3.18.0 as the
  minversion for tox

Signed-off-by: HeroicHitesh <email.hiteshkumar@gmail.com>
Change-Id: If2662e223ad52c8cae965eb6bdf880c1dce962f2
diff --git a/.gitignore b/.gitignore
index 7693c17..c3dab8f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,22 @@
 *.pyc
+
+# Packages
+*.egg*
+dist
+build
+
+# Unit test
+.tox/
+.stestr/
+.venv
+
+# pbr generates these
+AUTHORS
+ChangeLog
+
+# Editors
 *.swp
 *~
-.tox
-.testrepository
+
+# Files created by releasenotes build
+releasenotes/build