tests: Add tests for %-formatting of bytes.

This requires CPython3.5, to not require switching to it, just use .exp
file.

tests/basics/bytes_format_modulo.py

+# This test requires CPython3.5
+print(b"%%" % ())
+print(b"=%d=" % 1)
+print(b"=%d=%d=" % (1, 2))
+
+print(b"=%s=" % b"str")
+print(b"=%r=" % b"str")

tests/basics/bytes_format_modulo.py.exp

+b'%'
+b'=1='
+b'=1=2='
+b'=str='
+b"=b'str'="