Switch our python package management solution to poetry.

Mainly because it has better support for installing the packages from
different python versions.

It also has better dependency resolver than Pipenv. And supports modern
standard for python dependency management. This includes usage of
pyproject.toml for project specific configuration instead of per
tool conf files. See following links for details:
 https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/
 https://www.python.org/dev/peps/pep-0518/

docs/sourcetree.md

@@ -87,31 +87,29 @@ so manual installation of dependencies is not recommended.
 A single virtual environment with all dependencies is described in the single `Pipfile`.
 
 ### Prerequisites
-- Install Python 3.7 (the minimal supported version)
-    - Later version (e.g. 3.8) is ok if you don't write Python code
-    - You can install Python 3.7 separately, e.g.:
+- Install Python 3.7 (the minimal supported version) or greater.
+    - Our setup with poetry should work with newer python versions too. So feel free to open an issue with a `c/test-runner` label if something doesnt work as expected.
+    - If you have some trouble with other version you can resolve it by installing Python 3.7 separately, via pyenv or via system package manager e.g.:
       ```bash
       # In Ubuntu
       sudo add-apt-repository ppa:deadsnakes/ppa
       sudo apt update
       sudo apt install python3.7
       ```
-- Install `pipenv`
-    - Exact version of `pipenv` is not important, you can use Debian/Ubuntu package `pipenv`.
-- Install dependencies via either
-  * `pipenv --python 3.7 install --dev` if you will write Python code, or
-  * `pipenv install` if you only want to run Python scripts and don't have Python 3.7.
+- Install `poetry`
+    - Exact version of `poetry` is not important, see installation instructions available at poetry's [website](https://python-poetry.org/docs/#installation)`.
+- Install dependencies via `./scripts/pysync`. Note that CI uses Python 3.7 so if you have different version some linting tools can yield different result locally vs in the CI.
 
-Run `pipenv shell` to activate the virtual environment.
-Alternatively, use `pipenv run` to run a single command in the venv, e.g. `pipenv run pytest`.
+Run `poetry shell` to activate the virtual environment.
+Alternatively, use `poetry run` to run a single command in the venv, e.g. `poetry run pytest`.
 
 ### Obligatory checks
 We force code formatting via `yapf` and type hints via `mypy`.
 Run the following commands in the repository's root (next to `setup.cfg`):
 
 ```bash
-pipenv run yapf -ri .  # All code is reformatted
-pipenv run mypy .  # Ensure there are no typing errors
+poetry run yapf -ri .  # All code is reformatted
+poetry run mypy .  # Ensure there are no typing errors
 ```
 
 **WARNING**: do not run `mypy` from a directory other than the root of the repository.
@@ -123,17 +121,6 @@ Also consider:
 * Adding more type hints to your code to avoid `Any`.
 
 ### Changing dependencies
-You have to update `Pipfile.lock` if you have changed `Pipfile`:
+To add new package or change an existing one you can use `poetry add` or `poetry update` or edit `pyproject.toml` manually. Do not forget to run `poetry lock` in the latter case.
 
-```bash
-pipenv --python 3.7 install --dev  # Re-create venv for Python 3.7 and install recent pipenv inside
-pipenv run pipenv --version  # Should be at least 2021.5.29
-pipenv run pipenv lock  # Regenerate Pipfile.lock
-```
-
-As the minimal supported version is Python 3.7 and we use it in CI,
-you have to use a Python 3.7 environment when updating `Pipfile.lock`.
-Otherwise some back-compatibility packages will be missing.
-
-It is also important to run recent `pipenv`.
-Older versions remove markers from `Pipfile.lock`.
+More details are available in poetry's [documentation](https://python-poetry.org/docs/).