Stubs for psutil (#5776)

I generated these using stubgen and made various manual tweaks to fix issues
reported by stubtest.

Some of the submodules with underscore prefixes are used by open source code,
so I'm including them.

Various definitions are platform-specific. I added some sys.platform checks, but
it's hard to get these right. We may need to iterate on them later.

stubs/psutil/@tests/stubtest_allowlist.txt

@@ -0,0 +1,4 @@
+psutil._psbsd
+psutil._psosx
+psutil._psutil_windows
+psutil._pswindows