mypy: Don't use lazy ManyToManyField with implicit app label

Per
typeddjango/django-stubs#1802 (comment),
`ManyToManyField`s with lazy (string) models but no app label don't
work. In this case, we can just use the class directly, not the string,
so do that.

squaresdb/membership/models.py

@@ -235,9 +235,9 @@ class TSClass(models.Model):
     label = models.CharField(max_length=20)
     coordinator = models.ForeignKey(Person, on_delete=models.PROTECT,
                                     related_name='class_coord')
-    assistants = models.ManyToManyField('Person', through='TSClassAssist',
+    assistants = models.ManyToManyField(Person, through='TSClassAssist',
                                         related_name='class_assist')
-    students = models.ManyToManyField('Person', through='TSClassMember',
+    students = models.ManyToManyField(Person, through='TSClassMember',
                                       related_name='classes')
     start_date = models.DateField(null=True, blank=True)
     end_date = models.DateField(null=True, blank=True)