dev-python/pillow: fix broken tests

dev-python/pillow: fix broken tests

Starting with pytest 3.10, Pillow test suite breaks badly at collect
time. It seems to be related to the way pytest was invoked. Bringing
invocation closer to upstream's, that is, without specifying test unit
files, makes test run alright.

I fixed the invocation in 5.3 and, since 5.2 is already stabilized,
added a version constraint on pytest. The newly bumped v5.4 already has
a correct pytest invocation.

Signed-off-by: Virgil Dupras <vdupras@gentoo.org>
Package-Manager: Portage-2.3.51, Repoman-2.3.11