Fix #21121: Add archive of security issues.
Merge pull request #1641 from ubernostrum/security-issues-docs-21121

Fix #21121: Add archive of security issues.
Fixed a typo in the docs
Fixed two typos in the docs
Corrected markup problems in new security summary page.
Reworked security issue list to be per-issue, not per-release.
Fixed typo in exception message; refs #19414

Thanks Alexey Boriskin for the report.
Added __pycache__ to gitignore
Fixed #7557 -- Added type checking to Variable initialization.

Thanks tobias for the suggestion and boblefrag and saz for work on the
patch.
Fixed #21125 -- Removed support for cache URI syntax
Fixed #18744 -- Updated docstring to highlight limitations of NamedTemporaryFile

- Noted that this does not allow for reading and writing the same open
file in different processes under Windows.
- Noted that the keyword arguments to NamedTemporaryFile no longer
match the Python version.