Fix lower-constraint for PyMySQL

keystone does not have any lower constraint for PyMySQL so the
latest version 0.10.0 is picked by the job which is failing [1]
In OpenStack, PyMySQL upper constraint is .9.3 means that version
is tested not 0.10.0 [2] let's add PyMySQL lower constraint also
so that we test lower-constraint job with correct lower version.

[1]https://zuul.opendev.org/t/openstack/build/3077d96f4fff4b7985cb763d0635d471/log/job-output.txt#621
[2]https://github.com/openstack/requirements/blob/master/upper-constraints.txt#L384

Change-Id: I3834b3b34641c006c70614d5331d292c41f8a346
Closes-Bug: #1888886

lower-constraints.txt

@@ -49,6 +49,7 @@ python-ldap===3.0.0
 pymongo===3.0.2
 pysaml2==4.5.0
 PyJWT==1.6.1
+PyMySQL==0.7.6
 python-keystoneclient==3.8.0
 python-memcached===1.56
 pytz==2013.6