Ensure backwards-compatibility of bitstring lsb0

[micprog]

As some systems currently still have an older version of bitstring installed, this commit allows newer versions of dumpling to be backward-compatible with older bitstring versions.

[meggiman]

Hi @micprog,
Thanks for the PR. In my opinion however, it would make much more sense to force the required bitstring version to be installed (by adding a version >= specified in setup.cfg) than adding patches to the source code. That way, installing or upgrading dumpling with pip would automatically force you to install the right version of bitstring. Or are there any reasons I might overlook to insist on a source code patch?

[micprog]

In principle, I definitely agree with your approach. I am not sure if python version support (specifically python 3.6) is guaranteed for the updated bitstring package, but enforcing an updated python version is also reasonable.