.travis.yml: Force reinstall setuptools

The version of setuptools in the Travis virtualenv is current (44.1.1),
but tox .package get-build-requires fails with:

    running egg_info
    error: 'egg_base' must be a directory name (got `src`)

Which appears to be pypa/setuptools#1136
Perhaps Debian package (copied into virtualenv) is affected?  Reinstall
to avoid the issue.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

.travis.yml

@@ -23,7 +23,10 @@ jobs:
     env: TOXENV=pypy3
 
 install:
-- pip install --upgrade codecov coveralls setuptools tox wheel
+# Pre-installed version 44.1.1 affected by
+# https://github.com/pypa/setuptools/issues/1136
+- pip install --force setuptools
+- pip install --upgrade codecov coveralls tox wheel
 
 script:
 - tox -vv