In several places we were preparing repr of a string as '%s' which works
ok most of the time but leads to failure if string contains '\'
characters e.g. if it represents a path and we are running on windows:

        def test_pymain():
            from golang import b

            ....

            # -m <module>
            _ = pyout(['-m', 'hello', 'abc', 'def'], cwd=testdata)
            # realpath rewrites e.g. `local/lib -> lib` if local/lib is symlink
            hellopy = realpath(join(testdata, 'hello.py'))
    >       assert _ == b"hello\nworld\n['%s', 'abc', 'def']\n" % b(hellopy)
    E       assert b"hello\nworld\n['Z:\\\\home\\\\kirr\\\\src\\\\tools\\\\go\\\\pygo-win\\\\pygolang\\\\gpython\\\\testdata\\\\hello.py', 'abc', 'def'
    ]\n" == b"hello\nworld\n['Z:\\home\\kirr\\src\\tools\\go\\pygo-win\\pygolang\\gpython\\testdata\\hello.py', 'abc', 'def']\n"
    E         At index 17 diff: b'\\' != b'h'
    E         Full diff:
    E           (
    E         -  b"hello\nworld\n['Z:\\home\\kirr\\src\\tools\\go\\pygo-win\\pygolang\\gpytho"
    E         ?                                                                  ------------
    E         +  b"hello\nworld\n['Z:\\\\home\\\\kirr\\\\src\\\\tools\\\\go\\\\pygo-win\\\\pygo"
    E         ?                        ++    ++        ++   ++         ++  ++            ++
    E         -  b"n\\testdata\\hello.py', 'abc', 'def']\n",
    E         +  b"lang\\\\gpython\\\\testdata\\\\hello.py', 'abc', 'def']\n",
    E         ?    ++ ++++++++++++++            ++
    E           )

    gpython\gpython_test.py:183: AssertionError

-> Fix it by preparing repr via repr(...) properly.