issue-25872: Fix KeyError using linecache from multiple threads (pyth…

…onGH-18007)

The crash that this fixes occurs when using traceback and other modules from multiple threads; 
del cache[filename] can raise a KeyError.

Lib/linecache.py

@@ -71,10 +71,10 @@ def checkcache(filename=None):
  try:
  stat = os.stat(fullname)
  except OSError:
- del cache[filename]
+ cache.pop(filename, None)
  continue
  if size != stat.st_size or mtime != stat.st_mtime:
- del cache[filename]
+ cache.pop(filename, None)
 
 
 def updatecache(filename, module_globals=None):
@@ -84,7 +84,7 @@ def updatecache(filename, module_globals=None):
 
  if filename in cache:
  if len(cache[filename]) != 1:
- del cache[filename]
+ cache.pop(filename, None)
  if not filename or (filename.startswith('<') and filename.endswith('>')):
  return []