Fix lower-constraints job

oslotest 1.5.1 is restricted to pbr <1.0 which conflict
with other deps lower constarint for pbr.

Error:
The conflict is caused by:
    oslo-i18n 3.15.3 depends on pbr!=2.1.0 and >=2.0.0
    oslo-serialization 2.21.1 depends on pbr!=2.1.0 and >=2.0.0
    oslo-utils 4.5.0 depends on pbr!=2.1.0 and >=2.0.0
    oslotest 1.5.1 depends on pbr!=0.7, <1.0 and >=0.6

https: //zuul.opendev.org/t/openstack/build/cb2bce3b4e004601958b764dfd0737b8
Change-Id: I513262b85a57ce9f81a63bd778fdd7f7be27e485

lower-constraints.txt

@@ -2,7 +2,7 @@ oslo.config==5.2.0
 oslo.i18n==3.15.3
 enum34==1.0.4
 PrettyTable==0.7.1
-oslotest==1.5.1
+oslotest==3.5.0
 stestr==2.0.0
 oslo.serialization==2.21.1
 oslo.utils==4.5.0

test-requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 hacking>=3.0,<3.1.0 # Apache-2.0
-oslotest>=1.5.1
+oslotest>=3.5.0
 stestr>=2.0.0
 pre-commit>=2.6.0 # MIT
 oslo.serialization>=2.21.1 # Apache-2.0