where possible, open files using context managers

src/calibre/library/catalogs/epub_mobi_builder.py

@@ -2961,7 +2961,8 @@ class CatalogBuilder(object):
         left = max(int((MI_WIDTH - width) / 2.), 0)
         top = max(int((MI_HEIGHT - height) / 2.), 0)
         draw.text((left, top), text, fill=(0, 0, 0), font=font)
-        img.save(open(out_path, 'wb'), 'GIF')
+        with open(out_path, 'wb') as f:
+            img.save(f, 'GIF')
 
     def generate_ncx_header(self):
         """ Generate the basic NCX file.

src/calibre/library/database2.py

@@ -3517,10 +3517,9 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             self.set_metadata(id, mi, commit=True, ignore_errors=True)
             npath = self.run_import_plugins(path, format)
             format = os.path.splitext(npath)[-1].lower().replace('.', '').upper()
-            stream = lopen(npath, 'rb')
+            with lopen(npath, 'rb') as stream:
                 format = check_ebook_format(stream, format)
                 self.add_format(id, format, stream, index_is_id=True)
-            stream.close()
             postimport.append((id, format))
         self.conn.commit()
         self.data.refresh_ids(self, ids)  # Needed to update format list and size