Release htmlwidgets 1.6.3

[cpsievert]

Prepare for release:

• [x] git pull
• [x] Check current CRAN check results
• [x] Bump required R version in DESCRIPTION to 3.6
• [x] Polish NEWS
• [x] urlchecker::url_check()
• [ ] devtools::check(remote = TRUE, manual = TRUE)
• [ ] devtools::check_win_devel()
• [ ] revdepcheck::cloud_check()
• [ ] Update cran-comments.md
• [ ] git push
• [ ] Update static imports: staticimports::import()

Submit to CRAN:

• [ ] usethis::use_version('patch')
• [ ] devtools::submit_cran()
• [ ] Approve email

Wait for CRAN...

• [ ] Accepted :tada:
• [ ] usethis::use_github_release()
• [ ] usethis::use_dev_version(push = TRUE)