CI: pin pytest version on Python 3.5 (#26619)

Author: simonjayhawkins

ci/deps/azure-35-compat.yaml

@@ -26,5 +26,5 @@ dependencies:
   - pip
   - pip:
     # for python 3.5, pytest>=4.0.2 is not available in conda
-    - pytest>=4.0.2
+    - pytest==4.5.0
     - html5lib==1.0b2

ci/deps/azure-macos-35.yaml

@@ -25,7 +25,7 @@ dependencies:
   - pip:
     - python-dateutil==2.5.3
     # universal
-    - pytest>=4.0.2
+    - pytest==4.5.0
     - pytest-xdist
     - pytest-mock
     - hypothesis>=3.58.0