Travis CI failures on Python 2.7

[kevinoid]

Builds currently fail for Python 2.7 on Travis CI. For example, see Job 21.1 which fails with:

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

This appears to be pypa/setuptools#1136 which was fixed in 40.7.0 and again in 40.7.1. Since 44.1.1 (the most recent version with Python 2.7 support currently) is installed and the problem does not occur on GitHub Actions, I suspect it is a result of pypa/pip#6264, since Travis CI runs in a virtualenv.

It might be possible to work around, as in kxepal/setuptools-changelog@911d39e, but I would prefer to understand the cause first.

Since none of my current projects require support for Python 2.7 (which is EOL) nor Travis CI, this issue is low-priority. It will be kept open for future reference.