bpo-33660: Fix corner-case in PosixPath.resolve resulting in "//path"

[mbarkhau]

This PR is provisional, a bpo doesn't exist yet.

On POSIX, when Path.cwd() == Path("/"), then Path("path").resolve() == Path("//var")

This is because the normal case is that sep must be added between the base path and the relative path, as the prefix/base does not usually end with sep itself. In this respect, the root directory Path("/") is a corner-case that I believe was previously missed.

A consequence of this can be that the relative_to method raises an error, even though the intent was to just normalize different both absolute and relative paths to relative paths:

In [9]: Path.cwd()
Out[9]: PosixPath('/')

In [10]: Path("path").resolve().relative_to(Path("/"))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-10-02608bb55fa0> in <module>
----> 1 Path("path").resolve().relative_to(Path("/"))

~/miniconda3/envs/py38/lib/python3.8/pathlib.py in relative_to(self, *other)
    897         if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
    898             formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
--> 899             raise ValueError("{!r} does not start with {!r}"
    900                              .format(str(self), str(formatted)))
    901         return self._from_parsed_parts('', root if n == 1 else '',

ValueError: '//path' does not start with '/'

https://bugs.python.org/issue33660

[mbarkhau]

I see now this relates to https://bugs.python.org/issue33660

[mbarkhau]

Related: psf/black#1631

[ambv]

Thanks for this but I'd rather merge the original PR by Dong-Hee Na.