Add handling of PermissionError on Windows for an existing directory

That is my guess for what is happening in

    ________________________ test_DownloadDirectory_basic _________________________
    dandi\tests\test_download.py:1048: in test_DownloadDirectory_basic
        with DownloadDirectory(tmp_path, digests={}) as dl:
    dandi\download.py:889: in __exit__
        self.writefile.replace(self.filepath)
    C:\hostedtoolcache\windows\Python\3.10.11\x64\lib\pathlib.py:1247: in replace
        self._accessor.replace(self, target)
    E   PermissionError: [WinError 5] Access is denied: 'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-runneradmin\\pytest-0\\test_DownloadDirectory_basic0.dandidownload\\file' -> 'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-runneradmin\\pytest-0\\test_DownloadDirectory_basic0'

dandi/download.py

@@ -13,6 +13,7 @@
 from pathlib import Path
 import random
 from shutil import rmtree
+import sys
 from threading import Lock
 import time
 from types import TracebackType
@@ -887,7 +888,12 @@ def __exit__(
             if exc_type is None:
                 try:
                     self.writefile.replace(self.filepath)
-                except IsADirectoryError:
+                except (IsADirectoryError, PermissionError) as exc:
+                    if isinstance(exc, PermissionError):
+                        if not (
+                            sys.platform.startswith("win") and self.filepath.is_dir()
+                        ):
+                            raise
                     lgr.debug(
                         "Destination path %s is a directory; removing it and retrying",
                         self.filepath,