TST: Pin pytest and use python3.6-dbg.

Pytest-5.1.0 segfaults on python3.5-dbg. Because we will be
dropping 3.5 for NumPy 1.18 we might as well use Python3.6
for the debug testing. That cures the previous segfault,
but exposes another so we also pin pytest to 5.0.1.

.travis.yml

@@ -10,7 +10,6 @@ addons:
   apt:
     packages: &common_packages
       - gfortran
-      - libatlas-dev
       - libatlas-base-dev
       # Speedup builds, particularly when USE_CHROOT=1
       - eatmydata
@@ -33,12 +32,14 @@ env:
 python:
   - 3.5
   - 3.6
+  - 3.7
   - 3.8-dev
 matrix:
   include:
     - python: 3.7
       env: INSTALL_PICKLE5=1
-    - python: 3.5
+    - python: 3.6
+      dist: bionic
       env: USE_DEBUG=1
       addons:
         apt:

tools/travis-before-install.sh

@@ -36,6 +36,6 @@ fi
 
 
 pip install --upgrade pip setuptools
-pip install pytz cython pytest
+pip install pytz cython pytest==5.0.1
 if [ -n "$USE_ASV" ]; then pip install asv; fi
 popd