SG-43906 Remove Python 3.7/3.8 compatibility code

[stevelittlefish]

Summary

• Simplify the version guard in shotgun_api3/__init__.py to a single 3.9 check, matching the pattern already adopted in tk-core (c6204ccd).
• Remove the dead pre-3.8 branch in Shotgun._split_url(), which used the deprecated urllib.parse.splituser. setup.py already requires python_requires>=3.9.0, so both the old <3.7 hard-block/<3.9 warning split and the <3.8 URL-parsing branch were unreachable.

Test plan

• python3 -m pytest tests/test_unit.py — 34 passed
• Manual check of _split_url against the exact cases in tests/test_client.py::test_split_url (auth+port, port only, no auth, numeric-only auth) — all match expected output
• tests/test_client.py::test_split_url itself requires a live Shotgun site config not available in this environment — please run in CI

Part of epic SG-36195.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.83%. Comparing base (db056df) to head (49c0c3b).

Files with missing lines	Patch %	Lines
shotgun_api3/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   78.76%   78.83%   +0.06%     
==========================================
  Files           7        7              
  Lines        1851     1847       -4     
==========================================
- Hits         1458     1456       -2     
+ Misses        393      391       -2     

Flag	Coverage Δ
Linux	78.83% <90.90%> (+0.06%)	⬆️
Python-3.10	78.77% <90.90%> (+0.06%)	⬆️
Python-3.11	78.77% <90.90%> (+0.06%)	⬆️
Python-3.13	78.77% <90.90%> (+0.06%)	⬆️
Python-3.9	78.76% <90.90%> (+0.06%)	⬆️
Windows	78.89% <100.00%> (+0.03%)	⬆️
macOS	78.83% <90.90%> (+0.06%)	⬆️

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[carlos-villavicencio-adsk]

LGTM