try to fix the Travis `uvloop` problem

kirr:

this fixes "uvloop"-flavour CI failures where things were failing to
install with

    RuntimeError: uvloop requires Python 3.7 or greater
    An error occurred when trying to install /tmp/tmph6b7orkdget_dist/uvloop-0.15.2.tar.gz. Look above this message for any errors that were output by easy_install.
    While:
      Installing test.
      Getting distribution for 'uvloop>=0.5.1'.

(see e.g. https://travis-ci.com/github/zopefoundation/ZEO/jobs/498522609)

.travis.yml

@@ -39,7 +39,7 @@ matrix:
           python: 3.5
           env: ZEO4_SERVER=1
         - os: linux
-          python: 3.5
+          python: 3.7
           env: BUILOUT_OPTIONS=extra=,uvloop
 install:
     - pip install zc.buildout