[2.11.x] Restrict MarkupSafe < 2.1

[3nprob]

#1605 #1585 #1601 #1600 #1599 #1598 #1595 #1596 #1597 #1594 #1590 #1591 #1587 #1585

[3nprob]

CI seems to assume master as base - this should be mergable without conflicts

[ThiefMaster]

It's an old and unsupported version... we do not plan to make a release adding upper bounds; people would have to update their pin anyway, so they can simply add the pin on markupsafe instead..

[3nprob]

Those who did not add an explicit pin (very much like jinja itself) have had things not breaking without intervention. Those are now failing. Publishing this change will make it work againa without depending projects adding explicit pins.

[ThiefMaster]

People who did not pin Jinja would have gotten updates for both Jinja and Markupsafe -> no problem. So the only people are affected are these where only some of the deps are pinned...