Disable extended test dependencies also in Py3.8/9 since not all of them are compatible yet.

.travis.yml

@@ -114,7 +114,7 @@
 
 install:
   - python -c 'import sys; print("Python %s" % (sys.version,))'
-  - if [ -n "${TRAVIS_PYTHON_VERSION##*-dev}" ]; then pip install -r test-requirements.txt $( [ -z "${TRAVIS_PYTHON_VERSION##pypy*}" -o -z "${TRAVIS_PYTHON_VERSION##3.[478]*}" ] || echo " -r test-requirements-cpython.txt" ) ; fi
+  - if [ -n "${TRAVIS_PYTHON_VERSION##*-dev}" ]; then pip install -r test-requirements.txt $( [ -z "${TRAVIS_PYTHON_VERSION##pypy*}" -o -z "${TRAVIS_PYTHON_VERSION##3.[4789]*}" ] || echo " -r test-requirements-cpython.txt" ) ; fi
 #  - CFLAGS="-O2 -ggdb -Wall -Wextra $(python -c 'import sys; print("-fno-strict-aliasing" if sys.version_info[0] == 2 else "")')" python setup.py build
 
 before_script: ccache -s || true