4.2.0

Features:

• Add --ask option to pip-sync (#913). Thanks @georgek

Bug Fixes:

• Add compatibility with pip>=19.3 (#864, #904, #910, #912 and #915). Thanks @atugushev
• Ensure pip-compile --no-header <blank requirements.in> creates/overwrites requirements.txt (#909). Thanks @AndydeCleyre
• Fix pip-compile --upgrade-package removes «via» annotation (#931). Thanks @hramezani

Improved Documentation:

• Add info to README about layered requirements files and -c flag (#905). Thanks @jamescooke