Drop python 3.6 support

Co-authored-by: Branch Vincent <branchevincent@gmail.com>

* adds better windows shell support

* Updates per review
uses subprocess.run()
adds support for pwsh

* changes how command list is built

Add description of `source` property.

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>
Co-authored-by: Gabriel Galli <ggabriel96@hotmail.com>

No longer needed with current mypy / flake8

The current pre-commit version is not compatible with Python 3.10

updates:
- [github.com/pre-commit/pre-commit-hooks: v4.0.1 → v4.1.0](https://github.com/pre-commit/pre-commit-hooks/compare/v4.0.1...v4.1.0)
- [github.com/asottile/pyupgrade: v2.29.1 → v2.31.0](https://github.com/asottile/pyupgrade/compare/v2.29.1...v2.31.0)
- [github.com/pre-commit/mirrors-mypy: v0.910-1 → v0.931](https://github.com/pre-commit/mirrors-mypy/compare/v0.910-1...v0.931)

All test locally passed:

Results (50.84s):
     812 passed
       3 skipped

* Update flake8 settings

* Add poetry to isort namespace packages

* Ignore flake8 E501 for long test names

* Run automatic string formatting on tests

* Run automatic string formatting on src

* Fix typo "already exits"

* Ignore long URLs, hashes, and paths

* Fix in-line comments

* Fix in-line comments in tests

* Fix docstrings

* Ignore misc

* Fix f-strings in src

* Fix f-strings in tests

* Fix multi-line strings in src

* Fix multi-line strings in tests

* Fix some oddly autoformatted code

* Replace some strings by multi-line strings

* Revert namespace change

* Add black string processing to pre-commit

* Move black pre-commit config to pyproject.toml

* Autoformat install scripts

* Manual line wrapping for install scripts

* Disable black formatting for expected strings

* Replace strings by constant

* Ideas for refactoring

And also refering to issue #4961

* Respond to the review

Removed items() from os.environ and add blank lines

* Add link.is_wheel back

updates:
- [github.com/psf/black: 21.11b1 → 21.12b0](https://github.com/psf/black/compare/21.11b1...21.12b0)
- [github.com/pre-commit/pre-commit: v2.15.0 → v2.16.0](https://github.com/pre-commit/pre-commit/compare/v2.15.0...v2.16.0)

updates:
- [github.com/pre-commit/pre-commit: v2.13.0 → v2.15.0](https://github.com/pre-commit/pre-commit/compare/v2.13.0...v2.15.0)

* add pre-commit hooks for check & lock
* add pre-commit hook for export requirements.txt
* add pre-commit hooks to docs
* add pre-commit hook to check pre-commit-hooks.yaml file

Thanks @graingert and @asottile for pointing out the `files` constraint.
See discussion on #2511.

Co-authored-by: Thomas Grainger <tagrain@gmail.com>
Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>

* use new installer in README and website
* use double quotes for `pyproject.toml` example
* small improvement for installer note
* doc: use `python3` for invoking Python

Due to the wording in [1] the broader python tooling ecosystem
sometimes expects an exact match to the single line pkgutil extend_path
usage. This change replaces current pkgutil style namespace package
implementation with the one line implementation described in [1].

This change also causes an import order change as `poetry.core` is
correctly detected as a third-party package.

[1] https://packaging.python.org/guides/packaging-namespace-packages/#pkgutil-style-namespace-packages