Add pyproject toml file backended with pdm manager for python >= 3.9

[stankudrow]

Greetings.

The PR targets the following major goals:

• using pyproject.toml file as a single source for defining project metadata -> removing the setup.py script
• introducing pdm building, packaging and dependency manager
• declare python 3.9 a minimum version -> please refer to Python EOL dates
• adding linters (mypy + ruff) and test (pytest) dependency groups

It covered minor goals:

• the .gitignore got updated
• removing some python2 details (small ones, mostly replacing xrange with range utilities)
• adding py.typed file for mypy analysis
• the tests directory removed from the source code and is placed alongside
• the codec.py and types.py modules got linted and formatted with mypy and ruff packages
• the TestData got prefixed with underscore to remove pytest warning message

Notes:

• it does not cover all changes done in the PR Drop python2 support #118 about dropping python2 support (@danigm) .

[stankudrow]

@Parkayun , @Kludex , hello. Wouldn't you mind to review this PR ?

[stankudrow]

@Kludex , I saw the issue #123 , would you like to have it before this PR or is there any chance to bring GHA after having this PR merged?

[stankudrow]

@Kludex , https://app.travis-ci.com/github/py-bson - only bsonspec.org project is visible and no bson repository is present -> the README.rst has broken image and reference rendered incorrectly.

[stankudrow]

@Parkayun , @Kludex

[stankudrow]

@Kludex , regardless the 24 files being changed, this PR is not as big as it may seem.

Let's see:

• .gitignore - just updated according this Python template with slight changes (added ruff stuff, e.g,)
• travis.yml - no need to support versions older than py3.9
• pyproject.toml -> stores the project metadata, eliminates the need for setup.py and test.py scripts (all tests are gathered in a separate directory)
• and minor changes in this test file to remove pytest warnings and this one just occasionally.

In total, only 5 files to check.

UPD: running the pyupgrade over the code did not produce any new changes.

[stankudrow]

Closed in favour of the PR #133 .