wireprototypes: make `baseprotocolhandler` methods abstract

The documentation says it's an abstract base class, so let's enforce it.  The
`typing.Protocol` class is already an ABC, but it only prevents instantiation if
there are abstract attrs that are missing.  For example, from `hg debugshell`:

    >>> from mercurial import wireprototypes
    >>> x = wireprototypes.baseprotocolhandler()
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
    TypeError: Can't instantiate abstract class baseprotocolhandler with abstract method name
    >>> class fake(wireprototypes.baseprotocolhandler):
    ...     pass
    ...
    >>> x = fake()
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
    TypeError: Can't instantiate abstract class fake with abstract method name

That's great, but it doesn't protect against calling non-abstract methods at
runtime, rather it depends on the protocol type hint being added to method
signatures or class attrs, and then running a type checker to notice when an
instance is assigned that doesn't conform to the protocol.  We don't widely use
type hints yet, and do have a lot of class hierarchy in the repository area,
which could lead to surprises like this:

    >>> class fake(wireprototypes.baseprotocolhandler):
    ...     @property
    ...     def name(self) -> bytes:
    ...         return b'name'
    ...
    >>> z = fake()
    >>> z.client()
    >>> print(z.client())
    None

Oops.  That was supposed to return `bytes`.  So not only is a bad/unexpected
value returned, but it's one that violates the type hints (since the base
client() method will be annotated to return bytes).  With this change, we get:

    >>> from mercurial import wireprototypes
    >>> class fake(wireprototypes.baseprotocolhandler):
    ...     @property
    ...     def name(self) -> bytes:
    ...         return b'name'
    ...
    >>> x = fake()
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
    TypeError: Can't instantiate abstract class fake with abstract methods
        addcapabilities, checkperm, client, getargs, getpayload, getprotocaps, mayberedirectstdio

So this looks like a reasonable safety harness to me, and lets us catch problems
by running the standard tests while the type hints are being added, and pytype
is improved.  We should probably do this for all Protocol class methods that
don't supply a method implementation.

mercurial/wireprototypes.py

@@ -202,6 +202,7 class baseprotocolhandler(Protocol):
         Used for uniquely identifying the transport type.
         """
 
+    @abc.abstractmethod
     def getargs(self, args):
         """return the value for arguments in <args>
 
@@ -210,12 +211,14 class baseprotocolhandler(Protocol):
         a dict mapping argument name to value.
         """
 
+    @abc.abstractmethod
     def getprotocaps(self):
         """Returns the list of protocol-level capabilities of client
 
         Returns a list of capabilities as declared by the client for
         the current request (or connection for stateful protocol handlers)."""
 
+    @abc.abstractmethod
     def getpayload(self):
         """Provide a generator for the raw payload.
 
@@ -223,6 +226,7 class baseprotocolhandler(Protocol):
         processed.
         """
 
+    @abc.abstractmethod
     def mayberedirectstdio(self):
         """Context manager to possibly redirect stdio.
 
@@ -236,9 +240,11 class baseprotocolhandler(Protocol):
         won't be captured.
         """
 
+    @abc.abstractmethod
     def client(self):
         """Returns a string representation of this client (as bytes)."""
 
+    @abc.abstractmethod
     def addcapabilities(self, repo, caps):
         """Adds advertised capabilities specific to this protocol.
 
@@ -247,6 +253,7 class baseprotocolhandler(Protocol):
         Returns a list of capabilities. The passed in argument can be returned.
         """
 
+    @abc.abstractmethod
     def checkperm(self, perm):
         """Validate that the client has permissions to perform a request.