Make psutil reloadable on linux

For safety reasons, reloading a module does not reinitialize c
extensions. For this reason, `del <c_extension>` in `__init__.py`
yields the following on reimport:

```
Traceback (most recent call last):
  File "/src/test/_linux.py", line 479, in test_psutil_is_reloadable
    imp.reload(psutil)
  File "psutil/__init__.py", line 89, in <module>
    from . import _psutil_linux
ImportError: cannot import name _psutil_linux
```

As none of the other platform-specific extensions are removed from
the module dict post-import, we simply omit the `del` for linux.

psutil/__init__.py

@@ -107,7 +107,6 @@
             RLIMIT_SIGPENDING = _psutil_linux.RLIMIT_SIGPENDING
         except AttributeError:
             pass
-        del _psutil_linux
 
 elif sys.platform.startswith("win32"):
     from . import _pswindows as _psplatform

test/_linux.py

@@ -44,6 +44,10 @@
 from test_psutil import unittest
 from test_psutil import which
 
+if PY3:
+    import importlib as imp
+else:
+    import imp
 
 # procps-ng 3.3.10 changed the output format of free
 # and removed the 'buffers/cache line'
@@ -471,6 +475,9 @@ def test_procfs_path(self):
             psutil.PROCFS_PATH = "/proc"
             os.rmdir(tdir)
 
+    def test_psutil_is_reloadable(self):
+        imp.reload(psutil)
+
     # --- tests for specific kernel versions
 
     @unittest.skipUnless(