bpo-40059: Add tomllib (PEP-680) (GH-31498)

This adds a new standard library module, `tomllib`, for parsing TOML.

The implementation is based on Tomli (https://github.com/hukkin/tomli).

## Steps taken (converting `tomli` to `tomllib`)

- Move everything in `tomli:src/tomli` to `Lib/tomllib`. Exclude `py.typed`.
- Remove `__version__ = ...` line from `Lib/tomllib/__init__.py`
- Move everything in `tomli:tests` to `Lib/test/test_tomllib`. Exclude the following test data dirs recursively:
  - `tomli:tests/data/invalid/_external/`
  - `tomli:tests/data/valid/_external/`
- Create `Lib/test/test_tomllib/__main__.py`:

  ```python
  import unittest

  from . import load_tests


  unittest.main()
  ```


- Add the following to `Lib/test/test_tomllib/__init__.py`:

  ```python
  import os
  from test.support import load_package_tests

  def load_tests(*args):
      return load_package_tests(os.path.dirname(__file__), *args)
  ```

  Also change `import tomli as tomllib` to `import tomllib`.

- In `cpython/Lib/tomllib/_parser.py` replace `__fp` with `fp` and `__s` with
  `s`. Add the `/` to `load` and `loads` function signatures.

- Run `make regen-stdlib-module-names`

- Create `Doc/library/tomllib.rst` and reference it in `Doc/library/fileformats.rst`

Lib/test/test_tomllib/data/valid/array/array-subtables.json

@@ -0,0 +1,11 @@
+{"arr":
+  {"type":"array","value":
+    [
+      {"subtab":
+        {"val": {"type":"integer","value":"1"}
+        }
+      },
+      {"subtab": {"val": {"type":"integer","value":"2"}}}
+    ]
+  }
+}

Lib/test/test_tomllib/data/valid/array/array-subtables.toml

@@ -0,0 +1,7 @@
+[[arr]]
+[arr.subtab]
+val=1
+
+[[arr]]
+[arr.subtab]
+val=2

Lib/test/test_tomllib/data/valid/array/open-parent-table.json

@@ -0,0 +1,6 @@
+{
+  "parent-table": {
+    "arr": {"type":"array","value":[{},{}]},
+    "not-arr": {"type":"integer","value":"1"}
+  }
+}

Lib/test/test_tomllib/data/valid/array/open-parent-table.toml

@@ -0,0 +1,4 @@
+[[parent-table.arr]]
+[[parent-table.arr]]
+[parent-table]
+not-arr = 1