tox: remove Python 2.7 as it is broken

The test fails with:
======================================================================
ERROR: test_AddReaderToGroupException (test.test_PCSCExceptions.TestUtil)
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Traceback (most recent call last):
  File "/Users/rousseau/Documents/sc/pyscard/pyscard/test/test_PCSCExceptions.py", line 80, in test_AddReaderToGroupException
    "reader", "group")
  File "/Users/rousseau/Documents/sc/pyscard/pyscard/smartcard/pcsc/PCSCExceptions.py", line 64, in __init__
    super().__init__(message='Failed to add reader: ' + readername +
TypeError: super() takes at least 1 argument (0 given)

tox.ini

@@ -1,6 +1,5 @@
 [tox]
 envlist =
-    py27
     py38
     py39