fix(types): declare support for single questions

Without this patch, when calling `prompt` or `unsafe_prompt` with a `dict` on `questions`, mypy fails with:

    Argument 1 to "unsafe_prompt" has incompatible type "Dict[str, Any]"; expected "Iterable[Mapping[str, Any]]"

questionary/prompt.py

@@ -1,5 +1,5 @@
 from prompt_toolkit.output import ColorDepth
-from typing import Any, Dict, Optional, Iterable, Mapping
+from typing import Any, Dict, Optional, Iterable, Mapping, Union
 
 from questionary import utils
 from questionary.constants import DEFAULT_KBI_MESSAGE
@@ -13,7 +13,7 @@ def __init__(self, message: str, errors: Optional[BaseException] = None) -> None
 
 
 def prompt(
-    questions: Iterable[Mapping[str, Any]],
+    questions: Union[Mapping[str, Any], Iterable[Mapping[str, Any]]],,
     answers: Optional[Mapping[str, Any]] = None,
     patch_stdout: bool = False,
     true_color: bool = False,
@@ -74,7 +74,7 @@ def prompt(
 
 
 def unsafe_prompt(
-    questions: Iterable[Mapping[str, Any]],
+    questions: Union[Mapping[str, Any], Iterable[Mapping[str, Any]]],,
     answers: Optional[Mapping[str, Any]] = None,
     patch_stdout: bool = False,
     true_color: bool = False,