Popper

Highlights of this version:
* Popper can now itself run in docker (https://github.com/getpopper/popper/commit/c6dd58ff9c74a034a21f3c5b034e74e7ef13ff5f)
* New installer that setups popper to run in docker instead of on the host's python environment (https://github.com/getpopper/popper/commit/6284a524a44e34792b0083961cdc38bb0391a440)
* Several improvements to documentation: new getting started guide, diagram for "The Workspace" section, restructured README. (thanks to FernandaDguez !)
* Support for new `dir` step attribute (853, thanks to RicardoHE97!)
* New `popper help` command (854, thanks to fsr313!)
* Fix for ignoring git metadata for empty repositories, when the workspace resides in a git repository folder (878, thanks to JayjeetAtGithub!)
* The `GIT_REMOTE_ORIGIN_URL` environment variable is now defined in containers when the workflow runs inside a git repository folder (860, thanks to DanielOsunaV!)

This release contains a new implementation of the YAML syntax parser. In addition, the HCL syntax is deprecated and not supported anymore (2.5.0 was the last release supporting it).

Highlights of this release:
* Miscellaneous improvements to SLURM runner (799, 802)
* Versioning of library now uses `dunamai` (831)
* New parser implementation using `pykwalify` to validate YAML schema (ae7daa8)
* Code base now being linted/formatted with `black` (828)
* Examples of how to use Popper for CI of Go, C++, Javascript projects (41cd379)
* Move integration tests from Bash to Python (807, 810)
* Add `GIT_*` variables to the the container environment variables (823)
* Add `popper sh` command to run a step in interactive mode (843)

thanks to (in random order) vipulchhabra99, JayjeetAtGithub, FernandaDguez, nicolecastillo, edeediong, MJVNOR, michiboo for their contributors that made this release possible!

This release supports the new YAML format for popper workflows.

In this release:

* 698: sanitize container names.
* 699: transparent execution of actions in Singularity.
* 703: on CI executions, allow ability to pass any `popper run` flag via commit message
* 706: warn of unsupported features in Singularity
* 708: add CI steps for installing Singularity
* 711: drop `--recursive` flag
* 712: ability to inject pre- and post- workflows
* 724: remove `popper add`
* unit testing in Python (719, 722)
* several bug fixes (705, 718, 725).


Many thanks to JayjeetAtGithub, arshul and malept for their contributions!

New features in this release:

* 683 - Allow users to instruct popper to skip pulling images (`--skip-pull`) and cloning repos (`--skip-clone`)
* 485 - Support searching on a catalog available actions with `popper search`.
* 567 - Display information about an action (showing it's README content) with `popper info`.
* 628 - Run an action on workflow failure (`--on-failure` flag).

Thanks to JayjeetAtGithub for all his contributions!

New features in this release:

* Ability to run an action and all of its dependencies via a `--with-dependencies` flag (625).
* Skip actions from running by providing a `--skip` flag to `popper run` (626)
* Allow to run on any folder, not just those within a Git repo (647).
* Add ability to whitelist or skip certain workflows when executed on CI service (565).

Many thanks to JayjeetAtGithub for all his contributions!