Added pygments as optional dependency for AQMP TUI.
This is required for the upcoming syntax highlighting feature
in AQMP TUI.
The dependency has also been added in the devel optional group.

Added mypy 'ignore_missing_imports' for pygments since it does
not have any type stubs.

Signed-off-by: G S Niteesh Babu <niteesh.gs@gmail.com>