CI: limit pytest version on 3.6

closes pandas-dev#27406

Author: jreback

ci/deps/travis-36-cov.yaml

@@ -39,7 +39,7 @@ dependencies:
   - xlsxwriter
   - xlwt
   # universal
-  - pytest>=4.0.2
+  - pytest>=4.0.2,<5.0.0
   - pytest-xdist
   - pytest-cov
   - pytest-mock

ci/deps/travis-36-locale.yaml

@@ -34,7 +34,7 @@ dependencies:
   - xlsxwriter
   - xlwt
   # universal
-  - pytest>=4.0.2
+  - pytest>=4.0.2,<5.0.0
   - pytest-xdist
   - pytest-mock
   - pip