See issue 11390 for a discussion, but briefly: there is a bug in the readline library that causes it, on some systems, to emit a control sequence on stdout when readline is first initialized and TERM is set to xterm.  The bug can be avoided by setting TERM='', and since assert_python is often used to test output, the bogus characters can cause tests to fail on just some systems.  _assert_python should therefore set TERM='' unless the TERM environment variable is passed explicitly by the caller.

While this issue probably has minimal impact on the earlier python versions (where only doctest is likely to trigger this bug, because it calls pdb which calls readline), on 3.4 readline is always initialized, and therefore the bug is more likely to cause unexpected test failures.