Pip: protect_pip_from_modification_on_windows check breaks some valid use cases

[benoit-pierre]

Environment

• pip version: 19.3.1
• Python version: 3.6.8
• OS: Windows

Description

The protect_pip_from_modification_on_windows check can error out on some valid use cases (with no attempt to modify the calling pip's executable that would run foul of Windows limitations). E.g.:

• python src/pip install pip
• pip install -t target pip
• ...

The call when installing build requirements is probably impacted too.

Setuptools test suite is hit by this issue: see this code. Note that the issue does not seem to occur with Python 2.7.17: https://ci.appveyor.com/project/jaraco/setuptools/builds/28883044.

[pradyunsg]

We should revert #6864 until we can figure this out. This is a pretty major bug that affects anyone doing pip-related development on Windows (like @uranusjr).