Fix testsuite virtualenv

Fix sites ordering: add user site first, before system site.

Ensure `virtualenv.system_site_packages = True` (needed for testing
`--user`) does not result in adding the real site-packages' directory
to `sys.path`.

Release 10.0.0

News file formatting fix

Don't warn about script locations if --target is specified

fix PEP 518 support

* fix build environment handling when using PyPy
* do use the build environment for all build commands
* allow installing and building a wheel of a PEP 518 enabled
  package without prior installation of setuptools and/or wheels
* fix check for minimum supported requirements for PEP 518 support:
  - correctly handle complex requirements
  - both setuptools and wheels are needed

Fix how docs gets pip version, to work around an issue with readthedocs

Normalize case before checking for paths

As suggested by @pfmoore

Do not warn about script location if it's with sys.executable

BUG: ioctl(fd, termios.TIOCGWINSZ, ...) returns 8 bytes

Fix installing from Git with short SHA

Use all available distributions

Same as b5f56efd

Accept prerelease versions when checking conflicts

Fix name normalization used for install warnings

Fix PEP 518 support for VCS installs