I can reproduce the failure by arbitrarily adding a non-ascii character to one of the mailcap files searched for by the mailcap module (https://docs.python.org/3/library/mailcap.html#mailcap.getcaps), like $HOME/.mailcap, and by setting the process to an ascii-only locale (export LANG=C).  While this might be a somewhat unusual combination in practice, mailcap should be able to handle it.

By the way, this is a Python 3 problem since test_mailcap does not exist in Python 2.  2.7.6 might be the version of the system-supplied Python, but you appeared to be running with a Python you built.  Try

  ./python -V

to show the version of the Python you are using.