Update .travis.yml

Author: cclauss

.travis.yml

@@ -1,5 +1,11 @@
 os: linux
 dist: focal
+addons:
+  apt:
+    update: true
+    packages:
+      - python3-numpy  # TEMPORARY: Just for Python 3.9-rc2
+
 language: python
 python: 3.9-dev
 cache: pip
@@ -12,7 +18,6 @@ jobs:
         - black --check . || true 
         - flake8 --ignore=E203,W503 --max-complexity=25 --max-line-length=88 --statistics --count .
         - scripts/validate_filenames.py  # no uppercase, no spaces, in a directory
-        - apt-get install python3-numpy  # TEMPORARY: Just for Python 3.9-rc2
         - pip install -r requirements.txt  # fast fail on black, flake8, validate_filenames
       script:
         - mypy --ignore-missing-imports . || true  # https://github.com/python/mypy/issues/7907