`requirements-dev.txt`, but none of the others, was tracked with
Windows-style (CRLF) line endings. This appears to have been the
case since it was introduced in a1b7634 (as `dev-requirements.txt`)
and not to be intentional.

This only changes how it is stored in the repository. This does not
change `.gitattributes` (it is not forced to have LF line endings
if automatic line-ending conversions are configured in Git).

This resolves two warnings about Ruff configuration, by:

- No longer setting `ignore-init-module-imports = true` explicitly,
  which was deprecated since `ruff` 0.4.4. We primarily use `ruff`
  via `pre-commit`, for which this deprecation has applied since we
  upgraded the version in `.pre-commit-config.yaml` from 0.4.3 to
  0.6.0 in d1582d1 (gitpython-developers#1953).

  We continue to list `F401` ("Module imported but unused") as not
  automatically fixable, to avoid inadvertently removing imports
  that may be needed.

  See also:
  https://docs.astral.sh/ruff/settings/#lint_ignore-init-module-imports

- Rename the rule `TCH004` to `TC004`, since `TCH004` is the old
  name that may eventually be removed and that is deprecated since
  0.8.0. We upgraded `ruff` in `.pre-commit-config.yml` again in
  b7ce712 (gitpython-developers#2031), from 0.6.0 to 0.11.12, at which point this
  deprecation applied.

  See also https://astral.sh/blog/ruff-v0.8.0.

These changes make those configuration-related warnings go away,
and no new diagnostics (errors/warnings) are produced when running
`ruff check` or `pre-commit run --all-files`. No F401-related
diagnostics are triggered when testing with explicit
`ignore-init-module-imports = false`, in preview mode or otherwise.

In addition, this commit makes two changes that are not needed to
resolve warnings:

- Stop excluding `E203` ("Whitespace before ':'"). That diagnostic
  is no longer failing with the current code here in the current
  version of `ruff`, and code changes that would cause it to fail
  would likely be accidentally mis-st

- Add the version lower bound `>=0.8` for `ruff` in
  `requirements-dev.txt`. That file is rarely used, as noted in
  a8a73ff (gitpython-developers#1871), but as long as we have it, there may be a
  benefit to excluding dependency versions for which our
  configuration is no longer compatible. This is the only change in
  this commit outside of `pyproject.toml`.

This stops listing Ruff rule `E731` ("Do not assign a `lambda`
expression, use a `def`") as ignored, and fixes all occurrences of
it:

- Spacing is manually adjusted so that readability is not harmed,
  while still satisfying the current formatting conventions.

- Although the affected test modules do not currently use type
  annotations, the non-test modules do. Some of the lambdas already
  had type annotations, by annotating the variable itself with an
  expression formed by subscripting `Callable`. This change
  preserves them, converting them to paramter and return type
  annotations in the resulting `def`. Where such type annotations
  were absent (in lambdas in non-test modules), or partly absent,
  all missing annotations are added to the `def`.

- Unused paramters are prefixed with a `_`.

- `IndexFile.checkout` assigned a lambda to `make_exc`, whose body
  was somewhat difficult to read. Separately from converting it to
  a `def`, this refactors the expression in the `return` statement
  to use code like `(x, *ys)` in place of `(x,) + tuple(ys)`.

This change does not appear to have introduced (nor fixed) any
`mypy` errors.

This only affects lambdas that were assigned directly to variables.
Other lambda expressions remain unchanged.