spdx-expression-parse too strict with compound expressions including exception

[ahamid-fossa]

Expression	python spdx-tools 0.8.5	spdx-expression-parse.js 3.0.1/4.0.0
LicenseRef-foo WITH Classpath-exception-2.0	VALID (exit=0)	rejects "Syntax error"

per https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/ a simple expression can consist of a LicenseRef-<id-string> atom so this should be supported.

[kemitchell]

Would welcome a PR with a test, and of course a patch if you're up to it.

[ahamid-fossa]

#37