Try again to test with Python 3.8

.travis.yml

@@ -6,19 +6,5 @@ python:
     - 3.6
     - 3.7
     - 3.8
-script:
-    - python -Werror
-        -Wignore::DeprecationWarning:site
-        -Wignore::PendingDeprecationWarning:nose.importer
-        -Wignore::DeprecationWarning:nose.importer
-        -Wignore::DeprecationWarning:nose.suite
-        -Wignore::DeprecationWarning:asynctest.mock
-        -Wignore::DeprecationWarning:asynctest.case
-        -Wignore::DeprecationWarning:asynctest.helpers
-        `which nosetests` --with-coverage --cover-erase --cover-package aiokatcp
-    # The pinned version of flake8 doesn't work with Python 3.8
-    # (and it can't be upgraded until https://github.com/PyCQA/pyflakes/issues/445
-    # is fixed).
-    - if python --version | grep -v 'Python 3\.8\.'; then flake8 aiokatcp examples; fi
-    - mypy aiokatcp examples
+script: .travis/run.sh
 after_success: coveralls

.travis/run.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e -u
+
+py38="$(python -c 'import sys; print(sys.version_info >= (3, 8))')"
+if [ "$py38" = "True" ]; then
+    # Python 3.8 deprecates some features still needed to support 3.5
+    warn_args=""
+else
+    warn_args="
+        -Werror
+        -Wignore::DeprecationWarning:site
+        -Wignore::PendingDeprecationWarning:nose.importer
+        -Wignore::DeprecationWarning:nose.importer
+        -Wignore::DeprecationWarning:nose.suite"
+fi
+
+set -x
+python $warn_args `which nosetests` --with-coverage --cover-erase --cover-package aiokatcp
+# The pinned version of flake8 doesn't work with Python 3.8
+# (and it can't be upgraded until https://github.com/PyCQA/pyflakes/issues/445
+# is fixed).
+if [ "$py38" = "False" ]; then
+    flake8 aiokatcp examples
+fi
+mypy aiokatcp examples
+