version: support post versions

Add `head_only?` to Formula, replace `head?` in livecheck

build(deps-dev): bump github-pages from 207 to 208 in /docs

Bumps [github-pages](https://github.com/github/pages-gem) from 207 to 208.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](https://github.com/github/pages-gem/compare/v207...v208)
Signed-off-by: Ndependabot[bot] <support@github.com>

tab: fix nil modification time

sorbet: Update RBI files and prune files.yaml.

Autogenerated by [a scheduled GitHub Action](https://github.com/Homebrew/brew/blob/master/.github/workflows/sorbet.yml).

zsh completion: show --cask as an argument to list, outdated

style: require `tag` and `revision` for git urls

shared_audits: add freetube to GITHUB_PRERELEASE_ALLOWLIST

os/mac: updates for Xcode 12 on Catalina

pr-upload: add --no-commit flag

tap-new: enable bottles publishing

zsh completion: --cask in comments

--cask in upgrade

dev-cmd/pr-publish: add autosquash workflow

livecheck/pypi: minor edit to PyPI post version

bump-formula-pr: don't warn for virtualenv resource

options: handle `nil` array.

Fixes #8835

livecheck/pypi: allow post in pypi version

to GitHub Releases

pr-pull: fix signoff for linuxbrew-core