Use scandir on py3.5+ for less disk access on filename completion

On Python 3.5+, we can make use of scandir that not only list the
content of the directory as an iterator but caches some infomations (for
example, `is_dir()`; this avoid extra stats call to the underlying
filesytem and can be – according to pep 471 –  2x to 20 time faster
especially on NFS filesystem where stats call is expensive.

From a quick this is the only place where scandir would make sens, as
most other places only require the name.

Fixes 1381

jedi/_compatibility.py

@@ -27,6 +27,27 @@ is_py35 = is_py3 and sys.version_info[1] >= 5
 py_version = int(str(sys.version_info[0]) + str(sys.version_info[1]))
 
 
+if is_py35:
+    """
+    A super-minimal shim around listdir that behave like
+    scandir for the information we need.
+    """
+    class _DirEntry:
+
+        def __init__(self, name, basepath):
+            self.name = name
+            self.basepath = basepath
+
+        def is_dir(self):
+            path_for_name = os.path.join(self.basepath, self.name)
+            return os.path.isdir(path_for_name)
+
+    def scandir(dir):
+        return [_DirEntry(name, dir) for name in os.listdir(dir)]
+else:
+    from os import scandir
+
+
 class DummyFile(object):
     def __init__(self, loader, string):
         self.loader = loader

jedi/api/file_name.py

@@ -1,12 +1,36 @@
 import os
+import sys
 
-from jedi._compatibility import FileNotFoundError, force_unicode
+from jedi._compatibility import FileNotFoundError, force_unicode, scandir
 from jedi.evaluate.names import AbstractArbitraryName
 from jedi.api import classes
 from jedi.evaluate.helpers import get_str_or_none
 from jedi.parser_utils import get_string_quote
 
 
+if sys.version_info < (3,6) or True:
+    """
+    A super-minimal shim around listdir that behave like
+    scandir for the information we need.
+    """
+    class DirEntry:
+
+        def __init__(self, name, basepath):
+            self.name = name
+            self.basepath = basepath
+
+        def is_dir(self):
+            path_for_name = os.path.join(self.basepath, self.name)
+            return os.path.isdir(path_for_name)
+
+    def scandir(dir):
+        return [DirEntry(name, dir) for name in os.listdir(dir)]
+else:
+    from os import scandir
+
+
+
+
 def file_name_completions(evaluator, module_context, start_leaf, string,
                           like_name, call_signatures_callback, code_lines, position):
     # First we want to find out what can actually be changed as a name.
@@ -32,13 +56,13 @@ def file_name_completions(evaluator, module_context, start_leaf, string,
             string = to_be_added + string
     base_path = os.path.join(evaluator.project._path, string)
     try:
-        listed = os.listdir(base_path)
+        listed = scandir(base_path)
     except FileNotFoundError:
         return
-    for name in listed:
+    for entry in listed:
+        name = entry.name
         if name.startswith(must_start_with):
-            path_for_name = os.path.join(base_path, name)
-            if is_in_os_path_join or not os.path.isdir(path_for_name):
+            if is_in_os_path_join or not entry.is_dir():
                 if start_leaf.type == 'string':
                     quote = get_string_quote(start_leaf)
                 else: