deal with __MACOSX

Apple adds an additional hidden directory "__MACOSX" into the zip file which leads to the error
```
Traceback (most recent call last):
File "/braintree/home/fgeiger/check_files.py", line 43, in
check_files(sys.argv[1], sys.argv[2])
File "/braintree/home/fgeiger/check_files.py", line 22, in check_files
check_zip_file(zip)
File "/braintree/home/fgeiger/check_files.py", line 36, in check_zip_file
assert (len(os.listdir(work_dir)) == 1)
AssertionError
```
Resolve by ignoring all hidden directories.

Start with check_files.py, especially check_zip_file and the assertion shown in the traceback. Run the file-checking workflow against a zip containing the hidden __MACOSX directory; done means that archive is accepted without the assertion failing while the intended file check still works.

Written by the indexing model from the issue text.