argon2 as an optional dependency

In setups where passwords are not used, it is preferential to not have argon2 be an optional dependency:
- If not used, let us not import it
- argon2 has C-extensions, so it should ideally not be a hard requirement unless we can avoid it

It should still be a install requirement, as it is used by default, but pip allows you to side step that if you insist.

jupyter_server/auth/security.py

@@ -12,9 +12,6 @@
 import traceback
 import warnings
 
-import argon2
-import argon2.exceptions
-from argon2 import PasswordHasher
 from ipython_genutils.py3compat import cast_bytes, str_to_bytes, cast_unicode
 from traitlets.config import Config, ConfigFileNotFound, JSONFileConfigLoader
 from jupyter_core.paths import jupyter_config_dir
@@ -63,7 +60,8 @@ def passwd(passphrase=None, algorithm='argon2'):
             raise ValueError('No matching passwords found. Giving up.')
 
     if algorithm == 'argon2':
-        ph = PasswordHasher(
+        import argon2
+        ph = argon2.PasswordHasher(
             memory_cost=10240,
             time_cost=10,
             parallelism=8,
@@ -108,6 +106,8 @@ def passwd_check(hashed_passphrase, passphrase):
     True
     """
     if hashed_passphrase.startswith('argon2:'):
+        import argon2
+        import argon2.exceptions
         ph = argon2.PasswordHasher()
 
         try: