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

In newer mypy[1][2], `ManyToManyField`s with lazy (string) models but no
app label don't work. For the `to` model, we can just use `Person`
directly (and do on the line above), so do that. For the `through`
model, add the app label -- Django doesn't care, and it makes mypy
happy.

Also, in mypy, well-structured wildcard patterns match without a
submodule too, so simplify some config.

[1] typeddjango/django-stubs#1802 (comment),
[2] typeddjango/django-stubs#1719 (comment)
[3] https://mypy.readthedocs.io/en/stable/config_file.html#config-file-format

mypy.ini

@@ -9,5 +9,5 @@ ignore_missing_imports = True
 [mypy-social_core.*]
 ignore_missing_imports = True
 
-[mypy-social_django,social_django.*]
+[mypy-social_django.*]
 ignore_missing_imports = True

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='membership.TSClassAssist',
                                         related_name='class_assist')
-    students = models.ManyToManyField('Person', through='TSClassMember',
+    students = models.ManyToManyField(Person, through='membership.TSClassMember',
                                       related_name='classes')
     start_date = models.DateField(null=True, blank=True)
     end_date = models.DateField(null=True, blank=True)