pyinstaller: work around issue with setuptools > 44

see https://github.com/pypa/setuptools/issues/1963

scripts/borg.exe.spec

@@ -13,7 +13,7 @@ if is_win32:
     hiddenimports = []
 else:
     basepath = '/vagrant/borg/borg'
-    hiddenimports = ['borg.platform.posix']
+    hiddenimports = ['borg.platform.posix', 'pkg_resources.py2_warn', ]
 
 block_cipher = None