sys: various fixes (#3762)

* sys: simplify _getframe

* sys: mark positional-only args

* sys: fix platform availability

* sys: remove settscdump

This function is no longer documented in 3.6 and above (and doesn't
exist on master, was removed in 4fd64b9a6aba9e6e1a5d).
In 3.5 documentation it's mentioned that it's only available if Python
was compiled with a special flag:
https://docs.python.org/3.5/library/sys.html#sys.settscdump

* sys: update whitelists

tests/stubtest_whitelists/py38.txt

@@ -205,15 +205,6 @@ struct.iter_unpack
 struct.unpack
 struct.unpack_from
 sys.UnraisableHookArgs
-sys._getframe
-sys.call_tracing
-sys.exit
-sys.getrefcount
-sys.intern
-sys.setcheckinterval
-sys.setdlopenflags
-sys.setrecursionlimit
-sys.setswitchinterval
 threading.ExceptHookArgs
 time.CLOCK_PROF
 time.CLOCK_UPTIME

tests/stubtest_whitelists/py3_common.txt

@@ -757,7 +757,6 @@ sys.last_type
 sys.last_value
 sys.ps1
 sys.ps2
-sys.settscdump
 sys.tracebacklimit
 sysconfig.is_python_build
 sysconfig.parse_config_h