Python import breaks when importing submodule

I'm puzzled that I could not find a question on this yet, since this seems like a fairly common situation. I might have overlooked it. Similar questions (like this one) exist, but they all appear to have different goals and constraints.

I'm writing code that is using another project as a git submodule. The (simplified) situation looks like this:

.
├── A.py
└── sub
    ├── B.py
    └── C.py

The contents of the files are as follows;

A.py

import sub.B

print(sub.B.x)

B.py

import C

x = C.y * 2
if __name__ == '__main__':
    print(x)

C.py

y = 7

When I try to execute A.py, it's telling me:

  File "/Users/Joost/poc/sub/B.py", line 1, in <module>
    import C
ImportError: No module named 'C'

Naturally, it works just fine when I modify B.py to actually get C from sub.C. However, as sub is a submodule from a third party, I cannot do that. Also, it would break submodule functionality.

What is the proper way to deal with this?