[WIP] psutil.disk_swaps() (swap partitions)

[giampaolo]

This implements #1681, by adding a new psutil.disk_swaps() which lists swap partitions (and swap files).

# Linux
>>> import psutil
>>> psutil.disk_swaps()
[sdiskswaps(path='/dev/nvme0n1p3', total=11718652, used=2724, fstype='partition', priority=-2)]

# Windows
>>> import psutil
>>> psutil.disk_swaps()
[sdiskswaps(path='C:\\pagefile.sys', total=1118652, used=5721, peak=3453)]

Linux and Windows only for now, but at least macOS and/or FreeBSD would also be nice to have.
I'm still not 100% sure about the name.

UPDATE - added support for:

• Linux
• Windows
• FreeBSD
• OpenBSD

[giampaolo]

And here's some bikeshedding.
Current name:

• disk_swaps()
  • +1 because it's neutral about swap partitions vs. swap files (Linux can have both, Windows only has files (called pagefiles))
  • +1 /proc/swaps suggests someone already did the bikeshedding =)

Other possible names:

• swap_partitions():
  • +1 because of the swap prefix
  • -1 because we can also list swap files
• swap_disks():
  • +1 because we already have the "swap_" prefix of swap_memory()
  • -1 because it sounds like entries refer to an entire disk
• swap_locations():
  • +1 (reuse "swap_" prefix)