Support Python 3.6 and 3.7 again

.travis.yml

@@ -1,6 +1,8 @@
 dist: bionic
 language: python
 python:
+  - "3.6"
+  - "3.7"
   - "3.8"
 install:
   - "pip install -r requirements.txt"

setup.py

@@ -23,6 +23,8 @@ setuptools.setup(
     long_description_content_type="text/markdown",
     url="https://github.com/alanorth/csv-metadata-quality",
     classifiers=[
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
         "Operating System :: OS Independent",