[PATCH] Avoid DeprecationWarning on make check with Python >= 3.5

Hi,
As I saw Nathan add note about DeprecationWarning in run_test.py on
Subversion's Python 3 Support Status page yesterday, I've made a patch
to avoid warning.

I didn't read context around the code changed, I only rewrited it as the
example code[1] do.

[1] https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly

I've run it on Python 3.7.1 and it seems it can run Python test units
without DeprecationWarning.

Could anyone review this?

Thanks,

-- 
Yasuhito FUTATSUKI <futatuki_at_yf.bsdclub.org> / <futatuki_at_poem.co.jp>