fix: Accept path-like objects in subprocess arguments

* Also add subprocess test case using Path object

* uvloop already handles path-like cwd correctly, so I just
  copy-and-pasted the same logic to _init_args() method.

* The standard library uses "isinstance(obj, os.PathLike)" to check
  if an object is path-like, but os.PathLike exists as of Python 3.6.
  Since uvloop needs to support Python 3.5, we should use manual check
  for existence of the __fspath__ attribute.

* According to the official Python documentation:

  - https://docs.python.org/3/library/subprocess.html#subprocess.Popen
    The subprocess.Popen() constructor support path-like objects since
    Python 3.6 on POSIX and since Python 3.8 on Windows.

  - https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.subprocess_exec
    This page does not mention about path-like objects,
    but as of Python 3.8, it DOES support path-like objects.

tests/test_process.py

@@ -135,6 +135,21 @@ async def test():
 
         self.loop.run_until_complete(test())
 
+    @unittest.skipIf(sys.version_info < (3, 8, 0),
+                     "3.5 to 3.7 does not support path-like objects "
+                     "in the asyncio subprocess API")
+    def test_process_executable_2(self):
+        async def test():
+            proc = await asyncio.create_subprocess_exec(
+                pathlib.Path(sys.executable),
+                b'-W', b'ignore', b'-c', b'print("spam")',
+                stdout=subprocess.PIPE)
+
+            out, err = await proc.communicate()
+            self.assertEqual(out, b'spam\n')
+
+        self.loop.run_until_complete(test())
+
     def test_process_pid_1(self):
         async def test():
             prog = '''\

uvloop/handles/process.pyx

@@ -284,6 +284,13 @@ cdef class UVProcess(UVHandle):
         self.__args = args.copy()
         for i in range(an):
             arg = args[i]
+            try:
+                fspath = type(arg).__fspath__
+            except AttributeError:
+                pass
+            else:
+                arg = fspath(arg)
+
             if isinstance(arg, str):
                 self.__args[i] = PyUnicode_EncodeFSDefault(arg)
             elif not isinstance(arg, bytes):