Move mypy configuration into pyproject.toml

This is mainly to take advantage of the significantly more convenient
syntax for module overrides. For example, instead of:

    [mypy-synapse.api.*]
    disallow_untyped_defs = True

    [mypy-synapse.app.*]
    disallow_untyped_defs = True

    [mypy-synapse.crypto.*]
    disallow_untyped_defs = True

The pyproject.toml file allows:

    [[tool.mypy.overrides]]
    module = [
        "synapse.api.*",
        "synapse.app.*",
        "synapse.crypto.*",
    ]
    disallow_untyped_defs = true

Not a huge difference with two or three modules, but we list many more.

This is especially important as I'm about to set disallow_untyped_defs
at the global level and provide module-level opt-outs so that we require
typed definitions in all newly added code.

Switching to this format will save several hundred lines of repetitive
boilerplate and clarify the intent of each stanza.

It's also easier to keep things ordered, since it works to just pipe a
region to `sort`, while mypy.ini requires handling a 3-line structure.
Ordering helps keep these settings synced up with the files on disk.

Signed-off-by: Dan Callahan <danc@element.io>

changelog.d/11315.misc

@@ -0,0 +1 @@
+Port mypy.ini into pyproject.toml.