pytype: add some assertions to guide pytype through const dict with tricky typing structure

These assertions also make the code more explicit and slightly more readable.

Mute pytype warnings:

File "kallithea/tests/vcs/base.py", line 76, in setup_repo: No attribute 'path' on str [attribute-error]
In Union[kallithea.lib.vcs.nodes.FileNode, nothing, str]
File "kallithea/tests/vcs/base.py", line 76, in setup_repo: No attribute 'content' on str [attribute-error]
In Union[kallithea.lib.vcs.nodes.FileNode, nothing, str]