Use black and isort to reformat new code in tuf/api/*, like so:

```
black --line-length 80 api
isort --line-length 80 --profile black api
```

Besides downsizing the default line length to fit our Code Style
Guide no extra configuration is required.

Unified format according to black and isort will be enforced by
CI/CD in a future commit.

**Changes include:**
- Use double quotes instead of single quotes where feasible
- Re-wrap and re-indent long lines such as dict literals, function
  signatures and function calls, using hanging indent
  This will require an update in our Code Style Guide, which the
  benefits of using black seem worth.
  https://github.com/secure-systems-lab/code-style-guidelines/blob/master/python.md#indentation-and-line-continuation
- Update vertical and horizontal spacing
- Sort and wrap imports

See black and isort docs for details:
https://black.readthedocs.io/en/stable/the_black_code_style.html
https://pycqa.github.io/isort/docs/configuration/black_compatibility/

NOTE: If desired I can split commits by change and/or configure git
for this repo to ignore the corresponding revision(s) in git-blame.
https://github.com/psf/black#migrating-your-code-style-without-ruining-git-blame

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

The updated pylintrc is based on the Google Python Style Guide
pylint configuration at
https://google.github.io/styleguide/pylintrc with the following
differences:
- We don't list defaults which are applied anyway.
- We don't configure checks that seem unrelated to the code style
  guide.
- We don't disable any checks that are not in conflict with the
  current code or code style guide.

This has the advantage of a minimal configuration file which should
be easy to maintain and extend as required, e.g. if conflicting
code is added, or linting time becomes too long, due to unnecessary
checks.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Configure lint build in tox.ini to check if code in tuf/api/* is
formatted according to black and isort style rules:
https://black.readthedocs.io/en/stable/the_black_code_style.html
https://pycqa.github.io/isort/

In addition to our new style guide (theupdateframework#1128) and corresponding linter
configuration, requiring auto-formatting should help to further
reduce reviewing effort. The auto-formatter black was chosen for
the following reasons:
- It seems to be the most popular formatter in the Python ecosystem
- It is well documented including integration instructions with
  most of the tools we use (git, GitHub Actions, pylint, a range of
  editors, pyproject.toml theupdateframework#1161)
- It checks that the reformatted code produces a valid AST that is
  equivalent to the original
- It has almost no ways of customization, which means no
  customization effort required, and more (cross-project) style
  uniformity, lowering contribution barriers
- It converts single to double quotes, where reasonable, which is
  exactly what we recommend
- The style choices it makes seem generally reasonable and don't
 conflict with our style guide, except for favoring hanging over
 aligned indentation, which is the opposite of what we recommend.
 But we are willing to update the adapt our style guide.

Auto-format pre-commit configuration will be added in a subsequent
commit.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Add optional pre-commit configuration to install and run
auto-formatters when committing new code to tuf/api/*.
Auto-formatters include:
- trailing-whitespace
- end-of-file-fixer
- black
- isort

This commit also adds pre-commit to the dev dependencies
and updates the contributor instructions accordingly.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Black standardizes single to double quotes where feasible.
However, it doesn't seem to change double to single quotes nor adds
escape characters, as a consequence it skips standardization on
strings with mixed quotes.

Unfortunately, pylint's quote consistency check also doesn't detect
this, so the onus will remain on the reviewer in these cases.

**Unrelated changes**:
The commit still enables pylint's "check-quote-consistency" just in
case it can detect something the black doesn't.

The commit also fixes a syntax inconsistency in pylintrc.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

This reverts commit "Add basic pre-commit configuration for
tuf/api/*" (44aea45) in order to
reduce maintenance burdern:

- pre-commit really is a package manager, thus the packages (git
hooks) pulled in via pre-commit would need to be kept up-to-date
and securely so (sic!).

- pre-commit requires contributors to opt-in via "pre-commit
install" regardless, so we might as well ask contributors to add
and tend to the corresponding configuration file on their own.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Add cli snippet to run black and isort on the command line and
pointers to editor and pre-commit configuration to
docs/CONTRIBUTORS.rst.

Also add .pre-commit-config.yaml to .gitignore for independent
pre-commit configuration.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>