Avoid Python 3.7 pip 22.0.4 failure in Travis.

Author: okeuday

.travis.yml

@@ -9,6 +9,8 @@ python:
   - "3.9"
   - "pypy"
 #  - "pypy3"
+before_install:
+ - python -m pip install pip==20.1
 script: python setup.py test
 branches:
   only: