Adding missing close for open and replaced "fid=open(filename) fid.close()"

statements with the safer "with open(filename) as fid:" blocks.

Also removed unnecessary "try: ... finally: pass" statements
and refactored code from img2py function into _write_image and _replace_non_alphanumeric_with_underscore

Fixes #1574

docs/sphinx/_downloads/Notebook.1.py

@@ -53,9 +53,8 @@ class NotebookFrame(wx.Frame):
             path = dlg.GetPath()
 
             # Open the file as read-only and slurp its content
-            fid = open(path, 'rt')
-            text = fid.read()
-            fid.close()
+            with open(path, 'rt') as fid:
+                text = fid.read()
 
             # Create the notebook page as a wx.TextCtrl and
             # add it as a page of the wx.Notebook

docs/sphinx/rest_substitutions/snippets/python/contrib/Notebook.1.py

@@ -53,9 +53,8 @@ class NotebookFrame(wx.Frame):
             path = dlg.GetPath()
 
             # Open the file as read-only and slurp its content
-            fid = open(path, 'rt')
-            text = fid.read()
-            fid.close()
+            with open(path, 'rt') as fid:
+                text = fid.read()
 
             # Create the notebook page as a wx.TextCtrl and
             # add it as a page of the wx.Notebook