This commit adds a `tox.ini` file to the repository. It exposes a
few different test environments which can be used by the developer
for:

* Linting the code
* Checking Python modules dependencies
* Running unit tests with coverage report
* Running a static type checker

Instructions on how to use `tox` have been added to the `README.md`
file.

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>

This commit creates the `tests.yml` workflow. It is responsible
for running all the tests which are exposed by `tox.ini`:

* Lint with `flake8`
* Check dependencies with `pipdeptree`
* Unit tests and coverage with `pytest`
* Static type checking with `pyright`

`flake8` and `pipdeptree` run only once, with Ubuntu and with the
latest Python version.

On the other hand, the unit tests and static type checkings are
run using a matrix of operating systems and Python versions.

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>

`flake8` reported these issues, which are fixed by this commit:

```
setup.py:19:1: F401 'sys' imported but unused
setup.py:34:80: E501 line too long (84 > 79 characters)
setup.py:48:80: E501 line too long (84 > 79 characters)
```

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>

Tests were failing on GitHub Actions because the terminal size
is different from developers' laptop.

This commit fixes that issue by patching the return value of
`shutil`, so we fake the width.

Besides that, some other tests were failing on GitHub Actions
because the helper printed by `ArgumentParser` differs from
Python 3.10 onwards: it prints `options:` string instead of
`optional arguments:`.

This commit also fixes that issue with unit tests by conditionally
setting the expected value based on the Python minor version of
Python 3.

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>

`pyright` was reporting the following issues:

```
  pg_backup_api/server_operation.py:512:16 - error: Expression of type "tuple[str | bytearray | memoryview, int | Any]" cannot be assigned to return type "Tuple[str | None, int]" (reportGeneralTypeIssues)
pg_backup_api/utils.py
  pg_backup_api/utils.py:101:37 - error: Cannot access member "server_names" for type "None"
    Member "server_names" is unknown (reportGeneralTypeIssues)
  pg_backup_api/utils.py:102:34 - error: Cannot access member "get_server" for type "None"
    Member "get_server" is unknown (reportGeneralTypeIssues)
  pg_backup_api/utils.py:107:36 - error: "server" is not a known member of module "barman" (reportGeneralTypeIssues)
pg_backup_api/logic/utility_controller.py
  pg_backup_api/logic/utility_controller.py:57:33 - error: Cannot access member "server_names" for type "None"
    Member "server_names" is unknown (reportGeneralTypeIssues)
  pg_backup_api/logic/utility_controller.py:61:34 - error: Cannot access member "get_server" for type "None"
    Member "get_server" is unknown (reportGeneralTypeIssues)
  pg_backup_api/logic/utility_controller.py:69:37 - error: Cannot access member "servers_msg_list" for type "None"
    Member "servers_msg_list" is unknown (reportGeneralTypeIssues)
  pg_backup_api/logic/utility_controller.py:217:39 - error: "id" is not a known member of "None" (reportOptionalMemberAccess)
```

This commit fixes those issues by asserting that the variables
are instances of the expected types.

Note: we had to add `pyright: ignore` to a couple lines because
`barman` doesn't have type hints, otherwise `pyright` would keep
complaining about `Unknown` here in `pg-backup-api`.

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>

That test was failing on GitHub Actions when using Python 3.7
+ Flask 2.2.5. For some reason Flask returns `HTTP 400 Bad Request`
instead of `415 Unsupported Media Types` on such environment.

This commit works around that issue by changing the expected
value if such a combination is found.

References: BAR-133.

Signed-off-by: Israel Barth Rubio <[email protected]>