feat(retry): add Retry policy interface

Add a new interface, `sghi.retry.Retry` and a few of its
implementations. A retry policy allows an application to handle
transient failures such as when trying to connect to a network
resource or another service. This is achieved by transparently
repeating the failed operation until it succeeds or until some
criteria are met.

docs/conf.py

@@ -67,6 +67,7 @@
     ("py:class", "concurrent.futures._base.Executor"),  # sphinx can't find it
     ("py:class", "concurrent.futures._base.Future"),  # sphinx can't find it
     ("py:class", "sghi.dispatch._ST_contra"),  # private type annotations
+    ("py:class", "sghi.retry._RT"),  # private type annotations
     ("py:class", "sghi.task._IT"),  # private type annotations
     ("py:class", "sghi.task._OT"),  # private type annotations
     ("py:class", "sghi.utils.checkers._Comparable"),  # private type annotations

docs/index.rst

@@ -46,6 +46,7 @@ API Reference
      sghi.disposable
      sghi.exceptions
      sghi.registry
+     sghi.retry
      sghi.task
      sghi.typing
      sghi.utils

src/sghi/exceptions.py

@@ -24,3 +24,7 @@ def message(self) -> str | None:
             or ``None`` if one wasn't given.
         """
         return self._message
+
+
+class SGHITransientError(SGHIError):
+    """An error that indicates a temporary failure."""