…re. The older visitor relied on generic_visit() to be recursive. Where the provided generic_visit() is not recursive anymore, and moreover not called automatically when visiting unknow nodes! So what I'm saying here is that since #576 have been merged, we're not visiting the statements inside the 'orelse' field of Try and If nodes, same goes for 'finalbody' and 'handlers'.

This commit fixes that issue. The rationale is now the following: All statements in the 'orelse' block of IF nodes and statements in the except handlers of TRY nodes that would override a name already defined in the main 'body' (or TRY 'orelse' or 'finalbody') are ignored.

Meaning that in the context of the code below, 'ssl' would resolve to 'twisted.internet.ssl':

try:
    from twisted.internet import ssl as _ssl
except ImportError:
    ssl = None
else:
    ssl = _ssl

Properly add support for that to override_guard() function instead.

…:twisted/pydoctor into visit-try-orelse-finalbody-and-if-orelse

…YPE_CHECKING-value-for-a-module