Add typing to conda_content_trust.common (#76)

* Drop deprecated/commented code
* Combine is_a_signature with is_signature
* Rename is_a_signable to is_signable
* Use datetime parsing
* Add typing for:
    * HexString
    * GPGSignature & Signature
    * HexKey
    * Signable
    * BytesLike
    * natural int
    * str
    * GPGFingerprint
    * key
    * Delegation

conda_content_trust/authentication.py

@@ -28,10 +28,10 @@
     checkformat_gpg_signature,
     checkformat_hex_key,
     checkformat_signable,
-    is_a_signable,
     is_gpg_signature,
     is_hex_key,
     is_hex_signature,
+    is_signable,
     is_signature,
 )
 
@@ -342,7 +342,7 @@ def verify_signable(signable, authorized_pub_keys, threshold, gpg=False):
     #       we'll mostly have the hex strings on hand, but....
 
     # Argument validation
-    if not is_a_signable(signable):
+    if not is_signable(signable):
         raise TypeError(
             "verify_signable expects a signable dictionary.  "
             "Given argument failed the test."