Make version.py the single source of the version, and correct RELEASING.md

[MichaelGHSeg]

The version was duplicated, and only one copy was checked

pyproject.toml held version = "2.4.0" and segment/analytics/version.py held its own VERSION. The publish workflow validated the release tag against pyproject.toml only, but version.py is what the library actually reports at runtime:

msg["context"]["library"] = {"name": "analytics-python", "version": VERSION}   # client.py
"User-Agent": "analytics-python/" + VERSION                                      # request.py

So a stale version.py would publish a correctly numbered package while every event it ever sent was attributed to the previous release — and nothing would fail.

hatchling now reads the version from version.py via [tool.hatch.version], and the tag check validates against that same file. Verified locally: uv build resolves 2.4.0 from version.py and names the sdist accordingly.

RELEASING.md

Corrected against how 2.4.0 actually shipped:

• it claimed twine uploads with --trusted-publishing=always; no such flag exists and it aborted the first attempt
• the production environment requires review, so the publish job pauses — that step was missing
• bumping two files is no longer a step, since there is now one
• notes that the workflow runs from the tagged commit, which is why 2.4.0 had to be re-tagged rather than re-run