setup.cfg: Fix description/classifiers

setup.cfg

@@ -2,7 +2,7 @@
 name = pyprctl
 version = attr: pyprctl.__version__
 url = https://github.com/cptpcrd/pyprctl
-description = A partial reimplementation of psutil in pure Python using ctypes.
+description = An interface to Linux's prctl() syscall written in pure Python using ctypes.
 long_description = file: README.md
 long_description_content_type = text/markdown
 license = MIT
@@ -12,10 +12,6 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Operating System :: POSIX :: Linux
-    Operating System :: POSIX :: BSD :: OpenBSD
-    Operating System :: POSIX :: BSD :: NetBSD
-    Operating System :: POSIX :: BSD :: FreeBSD
-    Operating System :: MacOS :: MacOS X
 
 [options]
 packages = find: