Issue#130270: Add start parameter to mathmodule.c.h and mathmodule.c

[RSzipper]

Summary of Changes

• Added start parameter to math.sp (shortened from math_sumprod) for consistency with sum() and math.prod().
• Updated mathmodule.c.h and mathmodule.c to reflect the change.
• Modified function signatures and implementation to support an optional starting value.

Related Issue

Closes gh-130270

Additional Notes

• Ensures math.sp(p, q, start=x) starts summation from x instead of always defaulting to 0.
• Maintains strict length checking for input iterables.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[skirpichev]

See issue thread.