Stop pinning version of pbr

Follow up to "Migrate setup configuration to pyproject.toml" [1]

1. https://review.opendev.org/c/openstack/blazar-dashboard/+/972729

Change-Id: I6a2bfedfd125ead69390c2ebc5c808c5204600c7
Co-authored-by: Matt Crees <mattc@stackhpc.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

requirements.txt

@@ -1,4 +1,2 @@
-# PBR should always appear first
-pbr!=2.1.0,>=2.0.0 # Apache-2.0
 python-blazarclient>=1.0.1 # Apache-2.0
 horizon>=17.1.0  # Apache-2.0

setup.py

@@ -15,6 +15,4 @@
 
 import setuptools
 
-setuptools.setup(
-    setup_requires=['pbr>=2.0.0'],
-    pbr=True)
+setuptools.setup(pbr=True)