Dropdown menu navigation by the keyboard
Space or Enter both open a highlighted submenu where the current option does not have an action by itself. Right Arrow works when the current option does have an action, but you wish to open the submenu (I'm not aware of a single instance of such a case where it's not a bug).

This works regardless of whether you use ⌃F2 or clicking to open a menu. I am using macOS Sierra, but am not aware of this functionality missing from prior versions of macOS.

JetBrains apps such as PyCharm have an unfortunate bug where their submenus aren't just submenus since you can select the top level menu item too (try clicking on the highlighted item in your screenshot). Space prefers to select where possible, which won't expand the submenu. This is a specific bug with such apps.