[PATCH 5 of 8 v2] py3: make tests/md5sum.py use absolute_import

[Pulkit Goyal]

> This is only there to support Python versions < 2.5. Since we dropped
> support for anything < 2.6 now, the above could be simplified to:
>
>     from hashlib import md5

test-check-module-imports warn me on this :(

+  tests/md5sum.py:15: relative import of stdlib module
   Import cycle: hgext.largefiles.basestore ->
hgext.largefiles.localstore -> hgext.largefiles.basestore
   [1]

ERROR: test-check-module-imports.t output changed