Fix Python 3.5 travis build by pinning pip (#9949)

It looks like pip just dropped Python 3.5 support, but we are not quite
yet ready to do it (as long as there's an easy workaround).

Example failure here:
https://travis-ci.com/github/python/mypy/jobs/474676438

Author: JukkaL

.travis.yml

@@ -82,7 +82,8 @@ jobs:
   #   - EXTRA_ARGS=
 
 install:
-- pip install -U pip setuptools
+# pip 21.0 no longer works on Python 3.5
+- pip install -U pip==20.3.4 setuptools
 - pip install -U 'virtualenv<20'
 - pip install -U tox==3.20.1
 - python2 -m pip install --user -U typing