Bumpy required version of pytest to at least 4.6

[stuaxo]

Bump pytest version to 4.6 as required by pytest-cov.

[coveralls]

Coverage remained the same at 100.0% when pulling e73727e on stuaxo:patch-2 into 1aec93e on sgillies:master.

[stuaxo]

@sgillies I'm withdrawing this PR.

I assumed Affine would have native code parts, like Planar did, in which case it woul be worth checking Pypy compatibility.

Since they are not there this should just work in Pypy already.