interfaces: mark a few dirstate methods abstract

I'm not sure what's going on here, but when enabling pytype checking on this
package, it spits out the following errors:

    File "/mnt/c/Users/Matt/hg/mercurial/interfaces/dirstate.py", line 136, in changing_parents:
        bad return type [bad-return-type]
               Expected: Iterator
      Actually returned: None
      Attributes of protocol Iterator are not implemented on None: __next__
    File "/mnt/c/Users/Matt/hg/mercurial/interfaces/dirstate.py", line 145, in changing_files:
        bad return type [bad-return-type]
               Expected: Iterator
      Actually returned: None
      Attributes of protocol Iterator are not implemented on None: __next__

I guess technically that's true, because these methods only have a doc comment,
and don't explicitly return something or unconditionally raise an error.  The
strange thing is that both before and after this change, the *.pyi file that is
generated is unchanged, and contains:

      def changing_files(self, repo) -> contextlib._GeneratorContextManager: ...
      def changing_parents(self, repo) -> contextlib._GeneratorContextManager: ...

I'm not sure if the `@abstractmethod` should be the most inner or most outer
decoration.  We'll roll the dice with being the innermost, because that's how
`@abstractproperty` says it should be used in conjunction with `@property`.

We should probably make all of the methods without an actual body abstract, like
was done for some `mercurial.wireprototypes` classes in fd200f5bcaea.  But let's
hold off for now and do that enmass later.

mercurial/interfaces/dirstate.py

@@ -1,5 +1,6
 from __future__ import annotations
 
+import abc
 import contextlib
 import os
 import typing
@@ -127,6 +128,7 class idirstate(Protocol):
         """Callable for checking exec bits."""  # TODO: this comment looks stale
 
     @contextlib.contextmanager
+    @abc.abstractmethod
     def changing_parents(self, repo) -> Iterator:  # TODO: typehint this
         """Context manager for handling dirstate parents.
 
@@ -136,6 +138,7 class idirstate(Protocol):
         """
 
     @contextlib.contextmanager
+    @abc.abstractmethod
     def changing_files(self, repo) -> Iterator:  # TODO: typehint this
         """Context manager for handling dirstate files.