CI: pin pytest in minimum verions (#35274)

Author: simonjayhawkins

ci/deps/azure-36-minimum_versions.yaml

@@ -6,7 +6,7 @@ dependencies:
 
   # tools
   - cython=0.29.16
-  - pytest>=5.0.1, <6.0.0rc0
+  - pytest=5.0.1
   - pytest-xdist>=1.21
   - hypothesis>=3.58.0
   - pytest-azurepipelines