This script used to work, I assume my breakage is due to the python version.
```
Traceback (most recent call last):
File "/home/user/wspace/electrum/./contrib/add_cosigner", line 35, in <module>
version_spec = importlib.util.spec_from_file_location('version', 'electrum/version.py')
^^^^^^^^^^^^^^
AttributeError: module 'importlib' has no attribute 'util'
```
This allows the add_cosigner workflow to be done before the website links to new binaries.
(so Emzy or other builders can try to reproduce builds and add signatures before new binaries are fully live)
