verdi devel check-undesired-imports fails when tui extras is installed

[danielhollas]

verdi devel check-undesired-imports makes sure that we don't have heavy imports during verdi startup to keep the CLI snappy (especially for tab completion).

One of the expensive modules that it checks is asyncio. Unfortunately, when you install the tui extra, the trogon package that powers the tui functionality seems to use asyncio, and this makes the test fail.
(indeed, one of the TUIs current downsides is that it makes all CLI interactions slower, even if you don't use the TUI subcommand).

We should make the test more clever and don't check for asyncio import when tui extras is installed.