vsts fails to discover the current python

[gaborbernat]

see https://toxdev.visualstudio.com/tox/_build/results?buildId=454&view=logs

018-09-17T10:58:19.1541324Z __________________ test_tox_env_var_flags_inserted_isolated ___________________
2018-09-17T10:58:19.1541601Z [gw0] win32 -- Python 3.5.4 d:\a\1\s\.tox\py\scripts\python.exe
2018-09-17T10:58:19.1541733Z 
2018-09-17T10:58:19.1541970Z popen_env_test = <function popen_env_test.<locals>.func at 0x000001913AA75158>
2018-09-17T10:58:19.1543263Z 
2018-09-17T10:58:19.1543416Z     @pytest.mark.network
2018-09-17T10:58:19.1543668Z     def test_tox_env_var_flags_inserted_isolated(popen_env_test):
2018-09-17T10:58:19.1543898Z         cur_py = "py{}".format("".join(str(i) for i in sys.version_info[0:2]))
2018-09-17T10:58:19.1544629Z         res = popen_env_test(cur_py, True)
2018-09-17T10:58:19.1544962Z >       assert_popen_env(res)
2018-09-17T10:58:19.1545114Z 
2018-09-17T10:58:19.1545317Z cur_py     = 'py35'
2018-09-17T10:58:19.1545521Z popen_env_test = <function popen_env_test.<locals>.func at 0x000001913AA75158>
2018-09-17T10:58:19.1545984Z res        = <session.test_session.popen_env_test.<locals>.func.<locals>.IsolatedResult object at 0x000001913C055F28>
2018-09-17T10:58:19.1546142Z 
2018-09-17T10:58:19.1546313Z D:\a\1\s\tests\unit\session\test_session.py:299: 
2018-09-17T10:58:19.1546584Z _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2018-09-17T10:58:19.1547282Z 
2018-09-17T10:58:19.1547552Z res = <session.test_session.popen_env_test.<locals>.func.<locals>.IsolatedResult object at 0x000001913C055F28>
2018-09-17T10:58:19.1547718Z 
2018-09-17T10:58:19.1547869Z     def assert_popen_env(res):
2018-09-17T10:58:19.1548097Z >       assert res.result.ret == 0, res.result.out
2018-09-17T10:58:19.1548373Z E       AssertionError: .package create: C:\Users\VSSADM~1\AppData\Local\Temp\pytest-0\popen-gw0\test_tox_env_var_flags_inserte0\env_var_test\.tox\.package
2018-09-17T10:58:19.1548702Z E         .package installdeps: setuptools >= 35.0.2, setuptools_scm >= 2.0.0, <3
2018-09-17T10:58:19.1548977Z E         py35 create: C:\Users\VSSADM~1\AppData\Local\Temp\pytest-0\popen-gw0\test_tox_env_var_flags_inserte0\env_var_test\.tox\py35
2018-09-17T10:58:19.1549227Z E         ERROR: InterpreterNotFound: python3.5
2018-09-17T10:58:19.1549490Z E         ___________________________________ summary ___________________________________
2018-09-17T10:58:19.1549715Z E         ERROR:  py35: InterpreterNotFound: python3.5
2018-09-17T10:58:19.1549932Z E         
2018-09-17T10:58:19.1550104Z E       assert 1 == 0
2018-09-17T10:58:19.1550984Z E        +  where 1 = RunResult(ret=1, args=-e py35, out=\n.package create: C:\Users\VSSADM~1\AppData\Local\Temp\pytest-0\popen-gw0\test_tox_...____________________ summary ___________________________________\nERROR:  py35: InterpreterNotFound: python3.5\n\n, err=\n).ret
2018-09-17T10:58:19.1551591Z E        +    where RunResult(ret=1, args=-e py35, out=\n.package create: C:\Users\VSSADM~1\AppData\Local\Temp\pytest-0\popen-gw0\test_tox_...____________________ summary ___________________________________\nERROR:  py35: InterpreterNotFound: python3.5\n\n, err=\n) = <session.test_session.popen_env_test.<locals>.func.<locals>.IsolatedResult object at 0x000001913C055F28>.result
2018-09-17T10:58:19.1551993Z 
2018-09-17T10:58:19.1552219Z res        = <session.test_session.popen_env_test.<locals>.func.<locals>.IsolatedResult object at 0x000001913C055F28>
2018-09-17T10:58:19.1552451Z 

[gaborbernat]

Resolved via #995.