Adds missing pointer reference for slices along with test

[EvanOman]

Fixes #346

NOTE: The variadic test was failing for me before adding this test:

Click me

run cmd: [build -vm=python3 -output=/tmp/gopy-3250023487 -package-prefix  ./_examples/variadic]
go build -v ./_examples/variadic

--- Processing package: github.com/go-python/gopy/_examples/variadic ---

--- building package ---
gopy.test -test.paniconexit0 -test.timeout=10m0s
goimports -w variadic.go
go build -mod=mod -buildmode=c-shared -o variadic_go.so .
/home/evan/miniconda3/bin/python3 build.py
CGO_CFLAGS="-I/home/evan/miniconda3/include/python3.10" -fPIC -Ofast
CGO_LDFLAGS="-L/home/evan/miniconda3/lib" "-lpython3.10" -lcrypt -lpthread -ldl -lutil -lm -lm
go build -mod=mod -buildmode=c-shared -o _variadic.cpython-310-x86_64-linux-gnu.so .
running python3 test.py
FAIL
exit status 1
FAIL    github.com/go-python/gopy       123.954s

Is that test flaky for other people too?

However I did confirm that the new test I added failed before the fix and passed afterwards.

[richecr]

Thank you very much for the correction, I had this error.
I'm eagerly waiting for the release.

[rcoreilly]

Giving this a try now.

[rcoreilly]

looks like it works -- not sure how I missed the test before. Need to fix the CI testing here too someday. just had to run format on your slices.go file.