specifies matplotlib version to last verified correct version

Fixes 3d axis aspect bug in matplotlib>=3.1

correctly performs to_student on 9b977bd as promised by d9d5f080

corrects answer regex to include case with no quotes

to_student for 9b977bd

#69 and #70 for 12

to_student for 6d57476

#69 and #70 for 11

to_student for 53df26

adds back scripts directory to simplify workflow

#69 and #70 for 10, cleans up some text and code cells

#69 and #70 for 09

adds #69 and #70 for 08

#69 and #70 for 08

fixes formatting of bold quote