Removed psutil as a required pip dependency.

Moved to optional dependencies for `system` plugin.

It requires gcc, linux-headers and python-dev to be installed on the
system.

The `python-psutil` system package however will still be installed when
Platypush is installed through a package manager.

platypush/backend/http/__init__.py

@@ -10,8 +10,6 @@ from multiprocessing import Process
 from time import time
 from typing import Mapping, Optional
 
-import psutil
-
 from tornado.httpserver import HTTPServer
 from tornado.netutil import bind_sockets, bind_unix_socket
 from tornado.process import cpu_count, fork_processes
@@ -421,6 +419,14 @@ class HttpBackend(Backend):
         workers when the server terminates:
         https://github.com/tornadoweb/tornado/issues/1912.
         """
+        try:
+            import psutil
+        except ImportError:
+            self.logger.warning(
+                'Could not import psutil, hanging worker processes might remain active'
+            )
+            return
+
         parent_pid = (
             self._server_proc.pid
             if self._server_proc and self._server_proc.pid

platypush/plugins/system/manifest.json

@@ -3,22 +3,27 @@
     "events": {},
     "install": {
       "apk": [
-        "py3-py-cpuinfo"
+        "py3-py-cpuinfo",
+        "py3-psutil"
       ],
       "apt": [
-        "python3-cpuinfo"
+        "python3-cpuinfo",
+        "python3-psutil"
       ],
       "dnf": [
-        "python-cpuinfo"
+        "python-cpuinfo",
+        "python-psutil"
       ],
       "pacman": [
-        "python-py-cpuinfo"
+        "python-py-cpuinfo",
+        "python-psutil"
       ],
       "pip": [
-        "py-cpuinfo"
+        "py-cpuinfo",
+        "psutil"
       ]
     },
     "package": "platypush.plugins.system",
     "type": "plugin"
   }
-}
+}

requirements.txt

@@ -8,7 +8,6 @@ docutils
 flask
 marshmallow
 marshmallow_dataclass
-psutil
 python-dateutil
 python-magic
 pyyaml