Use PYTHONIOENCODING=ascii:strict instead of LC_ALL=C in TEST_ASCII on Travis
Use PYTHONIOENCODING=ascii:strict instead of LC_ALL=C in TEST_ASCII on Travis

This is because in Python 3.7, LC_ALL=C no longer results in Unicode errors
(PEP 538). Once we drop all Python versions lower than 3.7, we might consider
removing this test, as the environment it tests won't be very common.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy