try again to get a later pytest on travis python 3.6 and 3.7

diff --git a/.travis.yml b/.travis.yml
index c20c595193fba29259a7a3179756e751f0fb789b..b01b887c5ed2d918cb508c8ec3aa43e25b12e01e 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,7 @@ branches:
   except:
     - python3
 install:
- - pip install --update pytest
+ - pip install --upgrade pytest
  - pip install typing coverage pytest pytest-cov cryptography idna requests requests-toolbelt trio curio
 script:
  - pytest --cov=dns --cov-report=xml:coverage.xml