chore(release): prepare v6.1.0

Bump Version to 6.1.0 and add CHANGELOG.md covering the release.
6.1.0 is primarily a security update: it closes a {% filter %}
sandbox bypass, fixes two renderer panics reachable from template
input (removetags, {% ifchanged %}), and clarifies that pongo2's
ban-list and loader APIs do not provide a true sandbox.

v7.0.0-alpha.1 release

bump version 4.0.2

v3

Updates #61

v2 release