This commit modifies `Message.__getattribute__` to raise
`AttributeError` whenever an attempt is made to access an unset `oneof`
field. This provides several benefits over the current approach:

* There is no longer any risk of `betterproto` users accidentally
  relying on values of unset fields.

* Pattern matching with `match/case` on messages containing `oneof`
  groups is now supported. The following is now possible:
  ```
  @dataclasses.dataclass(eq=False, repr=False)
  class Test(betterproto.Message):
    x: int = betterproto.int32_field(1, group="g")
    y: str = betterproto.string_field(2, group="g")

  match Test(y="text"):
    case Test(x=v):
      print("x", v)
    case Test(y=v):
      print("y", v)
  ```
  Before this commit the code above would output `x 0` instead of
  `y text`, but now the output is `y text` as expected. The reason
  this works is because an `AttributeError` in a `case` pattern does
  not propagate and instead simply skips the `case`.

* We now have a type-checkable way to deconstruct `oneof`. When running
  `mypy` for the snippet above `v` has type `int` in the first `case`
  and type `str` in the second `case`. For versions of Python that do
  not support `match/case` (before 3.10) it is now possbile to use
  `try/except/else` blocks to achieve the same result:
  ```
  t = Test(y="text")
  try:
    v0: int = t.x
  except AttributeError:
    v1: str = t.y  # `oneof` contains `y`
  else:
    pass  # `oneof` contains `x`
  ```

This is a breaking change.

This is needed to allow formatting Python 3.10 pattern matching
syntax.