Add sudo apt-get install python-dev in .travis.yml

Author: medvedev1088

.travis.yml

@@ -8,6 +8,8 @@ matrix:
       env: TOX_POSARGS="-e py38"
     - python: "3.9"
       env: TOX_POSARGS="-e py39"
+before_install:
+  - sudo apt-get install python-dev
 install:
   - travis_retry pip install tox
 script: