Avoid outdated six

Avoids problem introduced by newer virtualenv which broke installation
of tox on systems that already had a six version >1.0 installed but
which was older than minimal needed by tox.

Author: ssbarnea

docs/changelog/1519.bugfix.rst

@@ -0,0 +1,2 @@
+Bump minimal six version needed to avoid using one incompatible with newer
+virtualenv. - by :user:`ssbarnea`

setup.cfg

@@ -43,7 +43,7 @@ install_requires =
     packaging >= 14
     pluggy >= 0.12.0, <1
     py >= 1.4.17, <2
-    six >= 1.0.0, <2
+    six >= 1.14.0, <2  # required when virtualenv>=20
     virtualenv >= 16.0.0
     toml >=0.9.4
     filelock >= 3.0.0, <4