Use PythonCall instead of PyCall for tests

[ToucheSir]

The former has caused and still continues to cause us some grief on CI. With more and more packages migrating to PythonCall and us not actually using it in Zygote's own source, I don't see much harm in doing this.

[Jay-sanjay]

@ToucheSir Hello sir, I am interested to solve this issue, can you please elaborate what I need to actually do here. I see this is related to writing some test cases, but I am still not sure.

[ToucheSir]

We have an existing test set at

Zygote.jl/test/features.jl

Lines 688 to 698 in f755127

	@testset "PyCall custom @adjoint" begin
	# Trigger Python install if required. Required for Buildkite CI!
	import Conda
	Conda.list()
	import PyCall
	math = PyCall.pyimport("math")
	pysin(x) = math.sin(x)
	Zygote.@adjoint pysin(x) = math.sin(x), (δ) -> (δ * math.cos(x), )
	@test Zygote.gradient(pysin, 1.5) == Zygote.gradient(sin, 1.5)
	end

which uses PyCall.jl. The goal would be to replace it with PythonCall.jl while keeping the code as close as possible to the original. Same function definition and rule, same inputs, etc.

[ToucheSir]

Fixed by #1485.