bpo-45766: Add direct proportion option to linear_regression(). (#29490)

* bpo-45766: Add direct proportion option to linear_regression().

* Update 2021-11-09-09-18-06.bpo-45766.dvbcMf.rst

* Use ellipsis to avoid round-off issues.

* Update Misc/NEWS.d/next/Library/2021-11-09-09-18-06.bpo-45766.dvbcMf.rst

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>

* Update signature in main docs

* Fix missing comma

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>