repo_management/defaults.py:
Add the IntEnum `RepoDbMemberType` to distinguish different members
(files) in a repository database (e.g. 'desc' or 'files' files).

repo_management/files.py:
Add iterator method `_read_db_file_member()` to yield the members of a
provided tarfile.TarFile.
The members are filtered by name using a regular expression, which can
be overridden.

tests/test_file.py:
Change `create_db_file()` to also create 'desc' and 'files' files below
the created temporary directories.
Assert, that all tested methods return a value.
Add test for `_read_db_file_member()`.

repo_management/files.py:
Add `_read_db_file()` to open a db file and return it as
tarfile.Tarfile. The method assumes gzip-compressed databases by
default, but can be used with other compression algorithms.

tests/test_file.py:
Add fixtures and tests for `_read_db_file()`.

repo_management/defaults.py:
Add the IntEnmu `FieldType` which tracks the required type for an
attribute when converting from 'desc' or 'files' files to pydantic
models.
Extend the DESC_JSON and FILES_JSON globals by introducing a dict per
key, that describes the name and the required type, when converting to
pydantic models.

repo_management/convert.py:
Rename `_files_data_to_dict()` to `_read_files_data_to_model()` and
return a pydantic model (`models.Files`) instead of a custom dict.
Add `_desc_data_to_model()` which allows for conversion of a 'desc' data
file (represented as io.StringIO) to a pydantic model
(`models.PackageDesc`).

tests/test_convert.py:
Add another test for raising RuntimeError in
`_read_files_data_to_model()`.
Add a parametrized test for `_desc_data_to_model()`.

repo_management/models.py:
Add pydantic models for all headers in 'desc' and 'files' files (found
in a repository database per pkgbase).
The models describe the attribute names in use when reading data from
the respective files and whether they are required or not.

poetry.lock:
Update the lock file to include the latest version of pydantic.

pyproject.toml:
Add pydantic to dependencies, as it will be used to validate
configuration and internal typing.

.gitlab-ci.yml:
Add Gitlab CI integration, that runs tox's linter and coverage targets
in a lint stage.

tests/test_convert.py:
Add initial tests for the convert module by adding a parametrized test
for `_files_data_to_dict()`.

repo_management/defaults.py:
Add the basic defaults module, providing data that translates keywords
found in the 'desc' and 'files' files of a binary repository database to
those used in JSON files, representing the same data.

repo_management/convert.py:
Add the convert module, and introduce the simple private method
(reimplementing a subset of the features from db2json)
`_files_data_to_dict()`, which translates the contents of a `files` file
to a typed dict.

tox.ini:
Add tox integration for testing (linting and coverage) in virtual
environments using poetry.

poetry.lock:
Add lock file with all dependencies currently used to test the project.

pyproject.toml:
Add a pyproject.toml for describing the project, configuring all
currently required dependencies and development dependencies.
The configuration adds entries for the code formatter black, the import
sorting tool isort and for the coverage report generation tool coverage.

.gitignore:
Add a ignore file for git, with many default entries for python projects
related files, that should never end up in version control.

.mypy.ini:
As mypy can not yet be configured through pyproject.toml, add a specific
configuration file for it.

.flake8:
As flake8 can not yet be configured in a pyproject.toml, use a specific
file for it.
Set the maximum complexity to 10 (can even be lower) and add default
exclusions for project unrelated files.

.editorconfig:
Add editorconfig integration, that covers .py, .rst and .yml files.

Signed-off-by: Alad Wenter <alad@archlinux.org>

Using 'black'. No config needed for it (yet).