Update 2019-10:
As stated in the issue tracker, Google has been working on a new Android SDK Command-line Tools release that runs on current JVMs (9, 10, 11+) and does not depend on deprecated JAXB EE modules!
You can download and use the new Android SDK Command-line Tools inside Android Studio or by manually downloading them from the Google servers:
For the latest versions check the URLs inside the repository.xml.
If you manually unpack the command line tools, take care of placing them in a subfolder inside your $ANDROID_HOME (e.g. $ANDROID_HOME/cmdline-tools/...).
Update 2021-03:
The latest stable command-line tools are also available at Googles Downloads-Website. These tools are newer than those linked above.