There are some same consts in a module. This commit merges them into
single instance. It reduces number of objects in memory after loading modules.
https://bugs.python.org/issue34100
Current support for hash-based bytecode files in `zipimport` is rather
sparse, which leads to test failures when the test suite is ran with
the ``SOURCE_DATE_EPOCH`` environment variable set.
This teaches zipimport to handle hash-based pycs properly.
