.travis.yml: Only test Python 3.8

The Unicode normalization feature requires Python 3.8 because the
unicodedata.is_normalized() function only appears there. If I find
another way to check if a string is normalized without normalizing
it first I will drop the requirements back down to Python 3.6.

See: https://docs.python.org/3/library/unicodedata.html

.travis.yml

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