Unclear deps about ctypeslib

[trolldbois]

Agreed. it's hard to keep up with the awesome llvm team :/

@blueforesticarus can you share your test case ?

[blueforesticarus]

This has been made alot clearer now that the pypi package depends on the clang package, and the README expressly mentions libclang-11.

Also, the package seems to be alot more forgiving than it used to; I've used the 2.3.0 release with clang 11 and clang 10 (and I think even clang 6) without any readily apparent problems.