v13.0 Branch Preparation

Pins version of Flake8
Updates requirements to point to branch

Change-Id: I24418b7f6b11db07a6ed5e91f153ecf67f86defa
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
diff --git a/requirements-dev.in b/requirements-dev.in
index c113bba..43d075c 100644
--- a/requirements-dev.in
+++ b/requirements-dev.in
@@ -14,4 +14,4 @@
 # limitations under the License.
 
 git+https://osm.etsi.org/gerrit/osm/common.git@master#egg=osm-common
--r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=master
+-r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v13.0