BF: allow for import of keyring.util.properties but with a Deprecation warning if used

[yarikoptic]

edit: no longer pertinent -- "shim" is complete and it seems that keyrings.alt tests run/pass (whatever is not skipped locally)

Unfortunately it is not a complete solution to #593 since there was likely
also a breakage in API of those properties since it would lead to e.g.

(git)lena:~/proj/misc/keyrings.alt[main]git
$> python -c 'import keyrings.alt.file'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/yoh/proj/misc/keyrings.alt/keyrings/alt/file.py", line 67, in <module>
	class EncryptedKeyring(Encrypted, Keyring):
  File "/home/yoh/proj/misc/keyrings.alt/keyrings/alt/file.py", line 73, in EncryptedKeyring
	@properties.ClassProperty
  File "/home/yoh/proj/misc/keyring/keyring/util/__init__.py", line 12, in __getattr__
	return getattr(_properties, a)
AttributeError: module 'keyring._properties_compat' has no attribute 'ClassProperty'. Did you mean: 'classproperty'?

so might need more compat shimming (allowing edits by maintainers for this PR, so push in if decide to take/improve it)

Closes #593

[jaraco]

I've taken a different approach in 83b6dd680bbc8a, just in case properties is expected to be an import target (from keyring.util.properties import ...).