Issue 13274:  Make the pure python code for heapq more closely match the C implementation for an undefined corner case.

Closes #7334: close source files on ElementTree.parse and iterparse (partial backport of issue #10093 from 3.2).

Issue #13237: Make the subprocess convenience helper documentation self-contained aside from the shared parameter description. Downgrade the pipe warnings at that level to notes (since those pipes are hidden, people are unlikely to even try it)

Issue #13237: remove some details that only apply to the 3.x version of this module and cross reference the relocated warning about the dangers of invoking the shell with untrusted input

Patch by Suman Saha.

before all tasks have completed.

Issue #13237: Rearrange subprocess module documentation to emphasise the convenience functions and commonly needed arguments