Fix expansion with @base

[anatoly-scherbakov]

• Guard active @base use in _expand_iri when base is not None
• Add regression test for @base vs property term expansion

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	102	8	92%	70, 129, 143, 189, 195–197, 221
iri_resolver.py	141	3	98%	307–308, 318
jsonld.py	2435	173	93%	274–278, 359, 396, 401, 411, 428–443, 479–480, 522, 530, 550, 558–559, 667–672, 752–753, 768–769, 829–834, 859–860, 875–876, 886–887, 912–913, 954, 964, 970–974, 985–986, 1019, 1028, 1035, 1100, 1123, 1145–1147, 1157–1160, 1175, 1198, 1210–1213, 1305, 1317–1320, 1340, 1369, 1402, 1406, 1443, 1486, 1504–1507, 1516, 1561, 1664–1671, 1698–1700, 1705, 1726–1729, 1905, 1926, 2354, 2363–2370, 2462, 2489, 2512, 2521, 3037, 3090, 3093, 3156, 3241, 3275, 3283, 3549, 3554, 3556, 3605, 3743–3747, 3813, 3850, 3913, 4133–4134, 4148, 4436, 4568, 4602, 4631–4634, 4660–4661, 4844, 4908, 4935, 5009, 5011, 5029, 5127, 5176, 5408, 5410, 5449, 5451, 5460, 5599, 5725, 5812, 5930–5944, 6003–6009, 6076, 6289, 6295–6299, 6338, 6341, 6431–6432, 6441
lib/pyld/documentloader
aiohttp.py	65	22	66%	28–38, 69, 75, 87, 99–107, 118–121, 138, 153–155
requests.py	41	5	88%	55, 69, 81–89
TOTAL	3156	215	93%

Tests	Skipped	Failures	Errors	Time
1653	59 💤	0 ❌	0 🔥	10.616s ⏱️

[davidlehn]

• Why are those dev libs now needed? Did the github testing systems change behavior?
• If that test is not in the main JSON-LD API test suite, please add it there immediately. The WG needs to agree on the behavior (I assume they will in this case) and have tests to ensure all implementations interoperate. That also removes the need to have the test duplicated here.
• If the above can not be done now for some reason, then before merging this, please add tracking issues in the json-ld api repo and here to add the test case and clean it up here.

[anatoly-scherbakov]

@davidlehn, thanks for the review!

• Updated the test comment as suggested 👍
• libxml2-dev / libxslt1-dev are needed when CI builds lxml from source on PyPy (no wheel for that combo). I did not check how it ran before, it failed for me, and I fixed it.

This bug is a very recent regression. I haven't found a test in the suite that would cover it. Which explains why the bug could appear.

PR in json-ld-api added: w3c/json-ld-api#686 — but, it is unclear when that can be merged. For that reason, I propose we merge this PR. I do not consider a redundant test a problem, provided that the test is spec compliant.