First try installing it normally:
sudo pip3 install picamraw
In case that fails with this error message (like for me):
Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple Collecting picamraw Could not install packages due to an EnvironmentError: 404 Client Error: Not Found for url: https://www.piwheels.org/simple/picamraw/
download it and install it manually: Copy the link of the most recent .whl file from https://pypi.org/project/picamraw/#files, download it using wget and install it using pip3, e.g.:
wget https://files.pythonhosted.org/packages/1e/47/4efb0d0ab5d40142424e7f3db545e276733a45bd7f7f9095919ef30c96b3/picamraw-1.2.64-py3-none-any.whl sudo pip3 install picamraw-1.2.64-py3-none-any.whl