stbt control: Refactoring: Use curses.ascii constants
stbt control: Refactoring: Use curses.ascii constants

Using `curses.ascii` constants instead of `ord(...)` calls makes the code
more readable and also makes `SPECIAL_CHARS` dictionary more consistent.

martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester