bpo-30897: Add is_mount() to pathlib.Path

[cooperlees]

• Add in is_mount() call to pathlib.Path similiar to os.path.ismount(path)
• Add tests for is_mount()

https://bugs.python.org/issue30897

[the-knights-who-say-ni]

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[cooperlees]

Getting a Windows VM to play with the error. All suggestions welcome!

[vstinner]

I removed "bpo-30891: " from the title, since http://bugs.python.org/issue30891 is unrelated.

[mlouielu]

would it be much easy to just return os.path.ismount(self) ?

[cooperlees]

Easier, possibly. But I still think I'll hit my current issue with Windows. No like function in the library hits the 'os' module and I am pretty sure Python core developers would want this library standalone where possible.

[cooperlees]

I may need to rebase - that appveyor failure looks unrelated to me.

[cooperlees]

Rebase got all checks to pass 💃