refactor(pytest): PytestNeotestAdapter (#24)

Having `NeotestResultCollector` (a pytest plugin) as a inner local
class would make the code a bit difficult to read due to quite much
indentation. This commit does refactoring on NeotestResultCollector
to make it a module-level class with a reference to NeotestAdapter.

This refactoring would make easier adding more pytest plugins
(e.g., debugger integration) in the future.

There should be no changes in behaviors.

neotest_python/base.py

@@ -1,5 +1,6 @@
+import abc
 from enum import Enum
-from typing import TYPE_CHECKING, Dict, List, Optional
+from typing import TYPE_CHECKING, Callable, Dict, List, Optional
 
 
 class NeotestResultStatus(str, Enum):
@@ -29,7 +30,8 @@ else:
     NeotestResult = Dict
 
 
-class NeotestAdapter:
+class NeotestAdapter(abc.ABC):
+
     def update_result(
         self, base: Optional[NeotestResult], update: NeotestResult
     ) -> NeotestResult:
@@ -40,3 +42,8 @@ class NeotestAdapter:
             "errors": (base.get("errors") or []) + (update.get("errors") or []) or None,
             "short": (base.get("short") or "") + (update.get("short") or ""),
         }
+
+    @abc.abstractmethod
+    def run(self, args: List[str], stream: Callable):
+        del args, stream
+        raise NotImplementedError