flake8: Fix F401 errors and minimize flake8 ignores

Author: jherland

.flake8

@@ -1,6 +1,6 @@
 [flake8]
 exclude = .??*, venv
-ignore = E203, E266, E501, W503, F403, F401
+ignore = E203, E266, E501
 max-line-length = 88
 max-complexity = 18
 select = B,C,E,F,W,T4,B9

utils/testvectors_lowcost.py

@@ -19,8 +19,6 @@
 """
 __license__ = "Apache 2.0"
 
-from datetime import datetime, timezone
-
 from dp3t.protocols.lowcost import next_day_key, generate_ephids_for_day
 
 KEY0 = bytes.fromhex("0000000000000000000000000000000000000000000000000000000000000000")

utils/testvectors_unlinkable.py

@@ -19,7 +19,7 @@
 """
 __license__ = "Apache 2.0"
 
-from binascii import hexlify, unhexlify
+from binascii import unhexlify
 from datetime import datetime, timezone
 
 from dp3t.protocols.unlinkable import (