stubgen: `--include-private` not effective for C modules

[kmaehashi]

Bug Report

--include-private option of stubgen is not effective for C modules.

To Reproduce

Generate a stub for C modules with private members, e.g., by cythonizing:

# foo.pyx
def _foo(): pass

Then run stubgen -m foo to generate a stub.

Expected Behavior

_foo() does not appear in the stub unless --include-private is specified.

Actual Behavior

_foo() always appear in the stub.

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.7.8
• Operating system and version: Ubuntu 18.04

[khameeteman]

I am experiencing the same issue, however this also happens when the modules are not compiled (mypy: 0.812)