[v7r2] Adapt Test_FilePlugin.py for https://bugs.python.org/issue43219

[chrisburr]

One of the unit tests has started failing as in Python 3.9.7+ shutil.copy was fixed to raise FileNotFoundError instead of IsADirectoryError, see:

• https://bugs.python.org/issue43219
• [3.9] bpo-43219: shutil.copyfile, raise a less confusing exception instead of IsADirectoryError (GH-27049) python/cpython#27082

It's an issue with the test code itself and not DIRAC.

Old behavour:

$ python --version
Python 3.9.5
$ python -c 'import shutil; shutil.copy2("/tmp/tmpajr883ky/subDir", "/tmp/tmpcu9phq73/test/subDir")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/cburr/miniconda3/envs/dirac-development/lib/python3.9/shutil.py", line 435, in copy2
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/home/cburr/miniconda3/envs/dirac-development/lib/python3.9/shutil.py", line 264, in copyfile
    with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
IsADirectoryError: [Errno 21] Is a directory: '/tmp/tmpajr883ky/subDir'

New behavour:

$ python --version
Python 3.9.7
$ python -c 'import shutil; shutil.copy2("/tmp/tmpajr883ky/subDir", "/tmp/tmpcu9phq73/test/subDir")'
Traceback (most recent call last):
  File "/home/cburr/miniconda3/envs/dirac-development-2/lib/python3.9/shutil.py", line 265, in copyfile
    with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
IsADirectoryError: [Errno 21] Is a directory: '/tmp/tmpajr883ky/subDir'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/cburr/miniconda3/envs/dirac-development-2/lib/python3.9/shutil.py", line 443, in copy2
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/home/cburr/miniconda3/envs/dirac-development-2/lib/python3.9/shutil.py", line 293, in copyfile
    raise FileNotFoundError(f'Directory does not exist: {dst}') from e
FileNotFoundError: Directory does not exist: /tmp/tmpcu9phq73/test/subDir/subDir

@chaen @atsareg Please can you review+merge+propagate this quickly the get the CI passing again.