Fix type annotations

[jku]

securesystemslib users can't use type checkers on the code that calls securesystemslib because we don't mark the library as typed (by adding py.typed file). We should do that:

• Almost all of the new API is well type annotated already
• Some older modules are not and I'm not too concerned if e.g. _gpg is not properly annotated even when we claim that the library is annotated
• At least hash is a core module though and should be annotated. It's tricky to do though and we might want to do simplify hashing #943 first

For reference, this is the list python-tuf would see currently:

tuf/api/_payload.py:671: error: Call to untyped function "digest" in typed context  [no-untyped-call]
tuf/api/_payload.py:675: error: Call to untyped function "digest_fileobject" in typed context  [no-untyped-call]
tuf/api/_payload.py:739: error: Call to untyped function "digest" in typed context  [no-untyped-call]
tuf/api/_payload.py:742: error: Call to untyped function "digest_fileobject" in typed context  [no-untyped-call]
tuf/api/_payload.py:1153: error: Call to untyped function "digest" in typed context  [no-untyped-call]
tuf/api/_payload.py:1272: error: Call to untyped function "digest" in typed context  [no-untyped-call]
tuf/api/metadata.py:236: error: Call to untyped function "FilesystemBackend" in typed context  [no-untyped-call]
tuf/api/metadata.py:337: error: Call to untyped function "FilesystemBackend" in typed context  [no-untyped-call]

So essentially two issues:

• looks like FilesystemBackend constructor just needs a return type
• the hash.digest*() issues are more annoying and might be a lot easier to fix if we simplify hashing #943

[jku]

https://github.com/jku/securesystemslib/tree/more-annotations contains everything else remaining except hash related fixes