Fix syntax error on build.py --python parameter evaluation

It is missing a parenthesis that causes a syntaxis error when trying to build with other interpreter::

    reingart@s5ultra:~/src/wxPython_Phoenix-trunk$ python build.py --python=/usr/bin/python3
    Command '"/usr/bin/python3" -c "import sys; print(sys.version[:3]"' failed with exit code 1.
      File "<string>", line 1
        import sys; print(sys.version[:3]
                                        ^
    SyntaxError: unexpected EOF while parsing