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/test_error.py

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2021 Taneli Hukkinen
+# Licensed to PSF under a Contributor Agreement.
+
+import unittest
+
+from . import tomllib
+
+
+class TestError(unittest.TestCase):
+    def test_line_and_col(self):
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads("val=.")
+        self.assertEqual(str(exc_info.exception), "Invalid value (at line 1, column 5)")
+
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads(".")
+        self.assertEqual(
+            str(exc_info.exception), "Invalid statement (at line 1, column 1)"
+        )
+
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads("\n\nval=.")
+        self.assertEqual(str(exc_info.exception), "Invalid value (at line 3, column 5)")
+
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads("\n\n.")
+        self.assertEqual(
+            str(exc_info.exception), "Invalid statement (at line 3, column 1)"
+        )
+
+    def test_missing_value(self):
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads("\n\nfwfw=")
+        self.assertEqual(str(exc_info.exception), "Invalid value (at end of document)")
+
+    def test_invalid_char_quotes(self):
+        with self.assertRaises(tomllib.TOMLDecodeError) as exc_info:
+            tomllib.loads("v = '\n'")
+        self.assertTrue(" '\\n' " in str(exc_info.exception))
+
+    def test_module_name(self):
+        self.assertEqual(tomllib.TOMLDecodeError().__module__, tomllib.__name__)
+
+    def test_invalid_parse_float(self):
+        def dict_returner(s: str) -> dict:
+            return {}
+
+        def list_returner(s: str) -> list:
+            return []
+
+        for invalid_parse_float in (dict_returner, list_returner):
+            with self.assertRaises(ValueError) as exc_info:
+                tomllib.loads("f=0.1", parse_float=invalid_parse_float)
+            self.assertEqual(
+                str(exc_info.exception), "parse_float must not return dicts or lists"
+            )