I found this behavior by chance:

  >>> with open('tokenize.py', 'w') as f:
  ...   f.write("print('This is so sad')")
  ...
  >>> import linecache
  This is so sad
  >>>

  path/of/python/Lib/linecache.py:
    import functools
    import sys
    import os
    import tokenize

    [...]

Meanwhile,

  >>> with open('functools.py', 'w') as f:
  ...     f.write("print('This is so sad')")
  ...
  >>> import linecache
  >>>

It seems for me to be broken: 'import' doesn't have clear precedence.

Did you restart python before the functools test?

Yes, I restarted and cleared directory before each test.

What is your environment - system and python version?

I get this on master checkout on windows 10:

Running Release|Win32 interpreter...
Python 3.10.0a0 (heads/master:46e19b61d3, Aug 12 2020, 18:02:36) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> with open('functools.py', 'w') as f:
...      f.write("print('This is so sad')")
...
23
>>> import linecache
This is so sad
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython\lib\linecache.py", line 11, in <module>
    import tokenize
  File "C:\Users\User\src\cpython\lib\tokenize.py", line 32, in <module>
    import re
  File "C:\Users\User\src\cpython\lib\re.py", line 315, in <module>
    @functools.lru_cache(_MAXCACHE)
AttributeError: module 'functools' has no attribute 'lru_cache'
>>> quit()
C:\Users\User\src\cpython>del functools.py

C:\Users\User\src\cpython>python
Running Release|Win32 interpreter...
Python 3.10.0a0 (heads/master:46e19b61d3, Aug 12 2020, 18:02:36) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> with open('tokenize.py', 'w') as f:
...      f.write("print('This is so sad')")
...
23
>>> import linecache
This is so sad
>>>

My environment is Ubuntu 18.04.4

Python version:
  Python 3.8.0 (default, Oct 28 2019, 16:14:01)
  [GCC 8.3.0] on linux

It is because module functools is already imported (you can see it in sys.modules), but module tokenize is not. It is likely due to module rlcompleter imported at startup on Linux, but not on Windows.