How fix Cartopy pip install error ‘Proj 4.9.0 must be installed.’

Problem:

When trying to install cartopy using e.g. pip3 install cartopy you see this error message:

Collecting cartopy
  Downloading Cartopy-0.19.0.post1.tar.gz (12.1 MB)
     |████████████████████████████████| 12.1 MB 13.2 MB/s 
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  ERROR: Command errored out with exit status 1:
   command: /usr/bin/python3 /tmp/tmp2q7tvpo8 get_requires_for_build_wheel /tmp/tmpfwd5htse
       cwd: /tmp/pip-install-dg2i313t/cartopy
  Complete output (1 lines):
  Proj 4.9.0 must be installed.
  ----------------------------------------
ERROR: Command errored out with exit status 1: /usr/bin/python3 /tmp/tmp2q7tvpo8 get_requires_for_build_wheel /tmp/tmpfwd5htse Check the logs for full command output.

Solution:

Install proj using e.g.

sudo apt -y install libproj-dev

and then install cartopy again using e.g.

sudo pip3 install cartopy