fix user install issue

[arnefmeyer]

Hi Jonny, a simple fix to get an editable autopilot installation. 'pip3 install -e .' results in "WARNING: The user site-packages directory is disabled. error: can't create or remove files in install directory". This now works with 'pip3 install --user -e .' (no sudo required).

[sneakers-the-rat]

thanks for this! I almost always work in venvs within pyenvs so I never see this error! After a few more stabilizing development moves I'll work on building an automated test suite, started an issue to keep track of things that need to be tested and added this to that: #45

Merging, and will include in next release (which should be today or the next few days)

for (perhaps just my) future reference, for further discussion see:

• Cannot install into user site directory with editable source. pypa/pip#7953
• pip ignores the site.ENABLE_USER_SITE value if user is set explicitly (asymetric with venv behaviour) pypa/pip#8794