Remove `six` as a test dependency (#15589)

mypyc/test-data/run-classes.test

diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index 268e07f..5961771 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -1370,9 +1370,8 @@
 
 [case testMetaclass]
 from meta import Meta
-import six
 
-class Nothing1(metaclass=Meta):
+class Nothing(metaclass=Meta):
     pass
 
 def ident(x): return x
@@ -1381,15 +1380,7 @@
 class Test:
     pass
 
-class Nothing2(six.with_metaclass(Meta, Test)):
-    pass
-
-@six.add_metaclass(Meta)
-class Nothing3:
-    pass
-
 [file meta.py]
-from typing import Any
 class Meta(type):
     def __new__(mcs, name, bases, dct):
         dct['X'] = 10
@@ -1397,10 +1388,8 @@
 
 
 [file driver.py]
-from native import Nothing1, Nothing2, Nothing3
-assert Nothing1.X == 10
-assert Nothing2.X == 10
-assert Nothing3.X == 10
+from native import Nothing
+assert Nothing.X == 10
 
 [case testPickling]
 from mypy_extensions import trait, mypyc_attr

test-requirements.txt

diff --git a/test-requirements.txt b/test-requirements.txt
index 6b046e1..6611878 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -14,5 +14,4 @@
 pytest-cov>=2.10.0
 ruff==0.0.272  # must match version in .pre-commit-config.yaml
 setuptools>=65.5.1
-six
-tomli>=1.1.0
+tomli>=1.1.0  # needed even on py311+ so the self check passes with --python-version 3.7