Removal of unnecessary dependency on py

[jayvdb]

pytest-xdist depends on pytest>=3.
pytest 3.0.0 requires py>=1.4.29
and latest pytest requires py>=1.4.33.
Thus it is unnecessary for pytest-xdist to include
a requirement on py>=1.4.22

Fixes #105

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[jayvdb]

Sorry if my changelog/foo entry is not correct. I am very happy to amend the commit to comply with whatever voodoo you are using. (and I think the PR template should explain what tool (towncrier?) is being used, so that the intelligent submitter can verify their changelog entry is correct).

[RonnyPfannschmidt]

thanks