Improve pre-commit config (#9563)

- Add a few more hooks. These are all very fast, and I've found them useful in other projects:
  - Autofixes:
    - `trailing-whitespace`: fixes trailing whitespace
    - `requirements-txt-fixer`: alphabetises items in `requirements.txt` files
    - `end-of-file-fixer`: makes sure every file ends with a single newline character
    - `mixed-line-ending`: Makes sure Windows users don't accidentally introduce CRLF line endings into a file that uses LF line endings
  - None-autofixes:
    - `check-yaml`: loads YAML files to validate syntax
    - `check-toml`: loads TOML files to validate syntax
    - `check-merge-conflict`: detects merge-conflict strings in files and blocks them from accidentally being committed
    - `check-case-conflict`: checks for files with names that would conflict on a case-insensitive filesystem like MacOS HFS+ or Windows FAT; blocks them from being committed.
  - Change the bot schedule to quarterly, to reduce noisy PRs
  - Change the `black` language target-version to Python 3.10, synching the setting here with the changes that were made to our `pyproject.toml` file in #7538

stubs/tensorflow/@tests/stubtest_allowlist.txt

@@ -1,4 +1,4 @@
-# Some methods are dynamically patched onto to instances as they 
+# Some methods are dynamically patched onto to instances as they
 # may depend on whether code is executed in graph/eager/v1/v2/etc.
 # Tensorflow supports multiple modes of execution which changes some
 # of the attributes/methods/even class hierachies.
@@ -17,6 +17,6 @@ tensorflow.Operation.__getattr__
 tensorflow.Variable.__getattr__
 # Internal undocumented API
 tensorflow.RaggedTensor.__init__
-# Has an undocumented extra argument that tf.Variable which acts like subclass 
+# Has an undocumented extra argument that tf.Variable which acts like subclass
 # (by dynamically patching tf.Tensor methods) does not preserve.
-tensorflow.Tensor.__getitem__
+tensorflow.Tensor.__getitem__