3.21.6

Enhancements:

• Add limited support for platform_system/sys_platform env markers in PyPI skeleton
• cran-skeleton: Adds a flag --no-comments to remove the instructional comments from the recipe files.

Bug fixes:

• When checking for circular dependencies in cross-compiling mode, build
  requirements are ignored now.
• Make sure symlinked directories are found in always_include_files
• Fix pinning expressions for prerelease builds

Contributors:

• @isuruf
• @mbargull
• @kenodegard
• @jdblischak
• @dbast
• @jezdez
• @ChristofKaufmann