remove pytest pin

Author: TomAugspurger

.travis.yml

@@ -82,7 +82,7 @@ before_install:
     - CONTAINER="wheels";
     - BUILD_COMMIT=$BUILD_COMMIT;
     - BUILD_DEPENDS="$NP_BUILD_DEP Cython==0.28.2"
-    - TEST_DEPENDS="$NP_TEST_DEP pytest==3.8.2 pytest-xdist pytest-mock moto hypothesis wheel==0.31.1"
+    - TEST_DEPENDS="$NP_TEST_DEP pytest pytest-xdist pytest-mock moto hypothesis wheel==0.31.1"
     - source multibuild/common_utils.sh
     - source multibuild/travis_steps.sh
     - before_install