Add `deadcode` utility to CI code checks

[ehildenb]

Currently we have some deadcode in the repository: #976 (comment).

We colud check on CI that no deadcode is kept in the repository in general, though it may not be as effective as we think because many functions are reported as dead because they're called with reflective dispatch, for example exec_*** functions in the __main__.py.

[ehildenb]

Ping @tothtamas28 about this too.