test: fix PEP484 no implicit optional argument types errors

Fix warnings for these files when ./test/lint/lint-python.py is run using
mypy 0.991 (released 11/2022) and later:

$ test/lint/lint-python.py
test/functional/test_framework/coverage.py:23: error: Incompatible default for argument "coverage_logfile" (default has type "None", argument has type "str")  [assignment]
test/functional/test_framework/coverage.py:23: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
test/functional/test_framework/util.py:318: error: Incompatible default for argument "timeout" (default has type "None", argument has type "int")  [assignment]
test/functional/test_framework/util.py:318: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
test/functional/test_framework/util.py:318: error: Incompatible default for argument "coveragedir" (default has type "None", argument has type "str")  [assignment]
test/functional/interface_rest.py:67: error: Incompatible default for argument "query_params" (default has type "None", argument has type "dict[str, Any]")  [assignment]
test/functional/interface_rest.py:67: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True

Verified using https://github.com/hauntsaninja/no_implicit_optional

For details, see:

https://mypy-lang.blogspot.com/2022/11/mypy-0990-released.html

test/functional/interface_rest.py

@@ -26,6 +26,7 @@ from test_framework.wallet import (
     MiniWallet,
     getnewdestination,
 )
+from typing import Optional
 
 
 INVALID_PARAM = "abc"
@@ -64,7 +65,7 @@ class RESTTest (BitcoinTestFramework):
             body: str = '',
             status: int = 200,
             ret_type: RetType = RetType.JSON,
-            query_params: typing.Dict[str, typing.Any] = None,
+            query_params: Optional[typing.Dict[str, typing.Any]] = None,
             ) -> typing.Union[http.client.HTTPResponse, bytes, str, None]:
         rest_uri = '/rest' + uri
         if req_type in ReqType:

test/functional/test_framework/coverage.py

@@ -11,6 +11,7 @@ testing.
 import os
 
 from .authproxy import AuthServiceProxy
+from typing import Optional
 
 REFERENCE_FILENAME = 'rpc_interface.txt'
 
@@ -20,7 +21,7 @@ class AuthServiceProxyWrapper():
     An object that wraps AuthServiceProxy to record specific RPC calls.
 
     """
-    def __init__(self, auth_service_proxy_instance: AuthServiceProxy, rpc_url: str, coverage_logfile: str=None):
+    def __init__(self, auth_service_proxy_instance: AuthServiceProxy, rpc_url: str, coverage_logfile: Optional[str]=None):
         """
         Kwargs:
             auth_service_proxy_instance: the instance being wrapped.

test/functional/test_framework/util.py

@@ -309,7 +309,7 @@ class PortSeed:
     n = None
 
 
-def get_rpc_proxy(url: str, node_number: int, *, timeout: int=None, coveragedir: str=None) -> coverage.AuthServiceProxyWrapper:
+def get_rpc_proxy(url: str, node_number: int, *, timeout: Optional[int]=None, coveragedir: Optional[str]=None) -> coverage.AuthServiceProxyWrapper:
     """
     Args:
         url: URL of the RPC server to call