AppVeyor always fails because Python 2.6 does not support set comprehensions

[cclauss]

Python 2.6 reached its end of life 4+ years ago.

[cclauss]

Traceback (most recent call last):
  File "c:\python26\Lib\runpy.py", line 122, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "c:\python26\Lib\runpy.py", line 34, in _run_code
    exec code in run_globals
  File "C:\projects\virtualenv\.tox\py\Scripts\pip.EXE\__main__.py", line 5, in <module>
  File "c:\projects\virtualenv\.tox\py\lib\site-packages\pip\_internal\__init__.py", line 42, in <module>
    from pip._internal import cmdoptions
  File "c:\projects\virtualenv\.tox\py\lib\site-packages\pip\_internal\cmdoptions.py", line 16, in <module>
    from pip._internal.index import (
  File "c:\projects\virtualenv\.tox\py\lib\site-packages\pip\_internal\index.py", line 526
    {str(c.version) for c in all_candidates},
                      ^
SyntaxError: invalid syntax

[hugovk]

@cclauss This can now be closed, Python 2.6 was dropped in #1175.

[gaborbernat]

we also moved to Azure