Drop support for end of life Python 3.6

[mkniewallner]

Before submitting your pull-request, make sure the following is done.

• Fork the repository and create your branch from master so that it can be merged easily.
• Update changelogs/next.md with your change (include reference to the issue & this PR).
• Make sure all of the significant new logic is covered by tests. Not applicable
• Make sure all quality checks are green (Gazr specification).

Description

Python 3.6 has reached end of life status, according to PEP 494.

Looking at last 6 months PyPI stats, Python 3.6 only accounts for about 0.25% of the total downloads:

$ pypistats python_minor tartiflette --start-date 2021-07

| category | percent | downloads |
|:---------|--------:|----------:|
| 3.8      |  60.11% |    20,529 |
| null     |  18.77% |     6,410 |
| 3.9      |   9.97% |     3,404 |
| 3.7      |   8.85% |     3,021 |
| 3.10     |   1.92% |       654 |
| 3.6      |   0.25% |        85 |
| 2.7      |   0.13% |        43 |
| 3.5      |   0.01% |         4 |
| Total    |         |    34,150 |

By requiring Python >= 3.7, the project could benefit from some features that are only available on 3.7+, like PEP 563 or dataclasses.

Note that cibuildwheel reads the minimum supported Python version from pyproject.toml, setup.cfg or setup.py (see documentation), so it should automatically stop building wheels for Python 3.6 if this PR is merged.

[mkniewallner]

Confirming that only wheels for Python 3.7+ would be built with the change: https://github.com/tartiflette/tartiflette/runs/4637180148?check_suite_focus=true