black/parser: optimize deepcopying nodes (#2611)

The implementation of the new backtracking logic depends heavily on deepcopying the current state of the parser before seeing one of the new keywords, which by default is an very expensive operations. On my system, formatting these 3 files takes 1.3 seconds.

```
 $ touch tests/data/pattern_matching_*; time python -m black -tpy310 tests/data/pattern_matching_*             19ms
All done! ✨ 🍰 ✨
3 files left unchanged.
python -m black -tpy310 tests/data/pattern_matching_*  2,09s user 0,04s system 157% cpu 1,357 total
```

which can be optimized 3X if we integrate the existing copying logic (`clone`) to the deepcopy system;
```
 $ touch tests/data/pattern_matching_*; time python -m black -tpy310 tests/data/pattern_matching_*              1ms
All done! ✨ 🍰 ✨
3 files left unchanged.
python -m black -tpy310 tests/data/pattern_matching_*  0,66s user 0,02s system 147% cpu 0,464 total
```

This still might have some potential, but that would be way trickier than this initial patch.

src/blib2to3/pytree.py

@@ -52,7 +52,7 @@ def type_repr(type_num: int) -> Union[Text, int]:
     return _type_reprs.setdefault(type_num, type_num)
 
 
-_P = TypeVar("_P")
+_P = TypeVar("_P", bound="Base")
 
 NL = Union["Node", "Leaf"]
 Context = Tuple[Text, Tuple[int, int]]
@@ -109,6 +109,9 @@ def _eq(self: _P, other: _P) -> bool:
         """
         raise NotImplementedError
 
+    def __deepcopy__(self: _P, memo: Any) -> _P:
+        return self.clone()
+
     def clone(self: _P) -> _P:
         """
         Return a cloned (deep) copy of self.

tests/data/pattern_matching_extras.py

@@ -1,3 +1,5 @@
+import match
+
 match something:
     case [a as b]:
         print(b)
@@ -7,3 +9,21 @@
         print(b)
     case Point(int() as x, int() as y):
         print(x, y)
+
+
+match = 1
+case: int = re.match(something)
+
+match re.match(case):
+    case type("match", match):
+        pass
+    case match:
+        pass
+
+
+def func(match: case, case: match) -> case:
+    match Something():
+        case another:
+            ...
+        case func(match, case):
+            ...