dev-python/meson-python: fix erroneous build dep

cython is only needed by the testsuite. Futhermore, cython is treated as
a shell tool (it is looked up using meson's compiler language handling),
so it never made sense to require PYTHON_USEDEP.

The PYTHON_USEDEP forces recompiling cython if you want to install
meson-python for multiple impls. It also means if you try for a pure
py3.13 system (which I'm trying to do for the binhost) but need a small
handful of 3.12 packages you get conflicts, because meson-python holds
back having a pure 3.13 experience for cython.

Fixes: 34bf7b6af8
Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>

dev-python/meson-python/meson-python-0.17.1.ebuild

@@ -32,8 +32,8 @@ RDEPEND="
 "
 BDEPEND="
 	${RDEPEND}
-	>=dev-python/cython-0.29.34[${PYTHON_USEDEP}]
 	test? (
+		>=dev-python/cython-0.29.34
 		>=dev-python/packaging-23.1[${PYTHON_USEDEP}]
 		dev-python/pytest-mock[${PYTHON_USEDEP}]
 		dev-vcs/git