Use python3.9 and yara-python-dex (#289)

* Use python3.9 and yara-python-dex

* Use focal

Author: CalebFenton

.travis.yml

@@ -1,16 +1,13 @@
 language: python
-dist: trusty
+dist: focal
 cache:
   pip: true
 #  directories:
 #    - $HOME/.cache/samples
 python:
-  - "3.6"
+  - "3.9"
 install:
-  - pip install --upgrade pip
-  - pip install wheel
-  - pip wheel --wheel-dir=/tmp/yara-python --build-option="build" --build-option="--enable-dex" git+https://github.com/VirusTotal/yara-python.git@v3.11.0
-  - pip install --no-index --find-links=/tmp/yara-python yara-python
+  - pip install --upgrade pip wheel setuptools
   - pip install -e .[dev,test]
   - pip install --upgrade pytest factory_boy
 script: