Clean Code: Python

Fix issues found by flake8-bugbear:

* Unused loop variables
* assert statements in non-test code
* mixing test code with production code
* mark one excessive test which takes ~10min to run
  as 'slow'

78_Sine_Wave/python/sinewave.py

@@ -46,7 +46,7 @@ radians = 0
 width = CENTER - 1
 
 # "Start long loop"
-for line_num in range(MAX_LINES):
+for _line_num in range(MAX_LINES):
 
     # Get string to display on this line
     curr_string = STRINGS[string_index]