Install curb 0.9.3 on MacOS Catalina

install_curb_0.9.3_on_macos_catalina.sh

# You'll probably also need to Install Ruby 2.3.x with OpenSSL 1.0 (on Catalina),
# follow this gist *before* you install curb: https://gist.github.com/sutanto/75a778f94edae116aeaa5ec56955ce58

# Recent Homebrew version no longer allow the `brew install formula` command, we'll need to create our own "custom" tap:
brew tap-new sutanto/homebrew-versions

# make sure that we don't use the "shallow" local homebrew repo
git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow

# extract the required curl version
brew extract --version 7.35.0 curl sutanto/homebrew-versions

# finally install it
# brew command below will fail with “Error: undefined method `strip' for :provided_by_osx:Symbol”, ignore this, the install is still completed successfully
brew install sutanto/homebrew-versions/curl@7.35.0

gem install curb -v '0.9.3' -- --with-openssl-dir=/usr/local/opt/openssl --with-curl-dir=/usr/local/opt/curl@7.35.0

@alexey, in the gist above, I put a link to https://gist.github.com/sutanto/75a778f94edae116aeaa5ec56955ce58 which described how to install Ruby 2.3 with OpenSSL 1.0.
I'll change the comment in the gist above so it'll be more obvious.