[#261]: Refactor document loaders to subclasses

[anatoly-scherbakov]

Closes #261

• Converts built-in requests and aiohttp document loaders to DocumentLoader subclasses while preserving factory call APIs.
• Adds factory tests for class-based loader instances.
• Ignores local .venv directories.

[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	2440	164	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, 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, 1505–1508, 1517, 1566, 1669–1676, 1703–1705, 1731–1734, 1910, 1948, 2381, 2390–2397, 2489, 2516, 2539, 2548, 3064, 3117, 3120, 3183, 3268, 3302, 3310, 3588, 3593, 3595, 3644, 3782–3786, 3852, 3889, 3952, 4187, 4475, 4609, 4643, 4672–4675, 4701–4702, 4885, 4949, 4976, 5050, 5052, 5070, 5168, 5217, 5449, 5451, 5490, 5492, 5501, 5640, 5768, 5855, 5975–5989, 6051, 6053, 6121, 6334, 6340–6344, 6383, 6386, 6476–6477, 6486
lib/pyld/documentloader
aiohttp.py	71	22	69%	29–39, 68, 74, 86, 98–106, 118–121, 138, 153–156
requests.py	49	5	90%	54, 68, 80–88
TOTAL	3209	206	94%

Tests	Skipped	Failures	Errors	Time
1682	52 💤	0 ❌	0 🔥	2m 20s ⏱️

[mielvds]

Thanks @anatoly-scherbakov ! Nothing much to add. I would still report this change in the changelog though.

[anatoly-scherbakov]

@mielvds Thanks! Added to the changelog and to the docs.