Sorting enzymes into classes

Eight enzymes are listed with the job each one does. A second list, JOBS, gives the class number for each kind of job. You will name the class of a job, count how many of the enzymes belong to each class, and list the ligases.

The idea. The six classes are numbered $1$ to $6$, and classes turns a number into a name. A ligase joins two molecules using ATP.

Your tasks.

1. Complete class_name(job) so that it returns the name of the class for a job.
2. Fill the dictionary counts with each class name and how many of the enzymes belong to it.
3. Set ligases to a list of the enzymes in the ligase class.