[3.15] Update SOURCE_URI for 3.15 branch

[hugovk]

No description provided.

[StanFromIreland]

updated here by python/release-tools/run_release.py

Did that break?

[hugovk]

It didn't raise during last month's 3.15.0b1 release:

✅  Checking CPython repository is clean
✅  Checking pyspecific
✅  Checking CPython repository is clean

Today's 3.15.0b2 release had:

✅  Checking CPython repository is clean
💥  Checking pyspecific
Traceback (most recent call last):
  File "/Users/hugo/github/release-tools/run_release.py", line 1504, in <module>
    main()
    ~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 1500, in main
    automata.run()
    ~~~~~~~~~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 255, in run
    raise e from None
  File "/Users/hugo/github/release-tools/run_release.py", line 252, in run
    self.current_task(self.db)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/hugo/github/release-tools/release.py", line 151, in __call__
    return getattr(self, "function")(db)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/Users/hugo/github/release-tools/run_release.py", line 553, in check_pyspecific
    raise ReleaseException(
    ...<3 lines>...
    )
ReleaseException: SOURCE_URI is incorrect (it needs changing before beta 1):
expected: SOURCE_URI = 'https://github.com/python/cpython/tree/3.15/%s'
got     : SOURCE_URI = 'https://github.com/python/cpython/tree/main/%s'

For comparison, last year's 3.14.0b1:

✅  Checking Git repository is clean
💥  Checking pyspecific
Traceback (most recent call last):
  File "/Users/hugo/github/release-tools/run_release.py", line 1412, in <module>
    main()
  File "/Users/hugo/github/release-tools/run_release.py", line 1408, in main
    automata.run()
  File "/Users/hugo/github/release-tools/run_release.py", line 273, in run
    raise e from None
  File "/Users/hugo/github/release-tools/run_release.py", line 270, in run
    self.current_task(self.db)
  File "/Users/hugo/github/release-tools/release.py", line 126, in __call__
    return getattr(self, "function")(db)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/hugo/github/release-tools/run_release.py", line 536, in check_pyspecific
    raise ReleaseException("SOURCE_URI is incorrect")
ReleaseException: SOURCE_URI is incorrect

Can look into that later.