Most distributed versions of Python include or have an option to include install the Python test suite. This is included in the main Python Makefile (Makefile.pre.in) make install step. Unfortunately, test directories to be installed need to be explicitly added to the Makefile so it is easy to overlook such a change when only running tests from a development build directory. The devguide should show how to safely test install Python and run tests with it. For most Unixy ststems, that involves something like:

./configure --prefix=/tmp/py
make -j3
make install
/tmp/py/bin/python3 -m test -w -uall -j3 ...