* enable strict mypy checking
* treat poetry.core as untyped
* python 3.7 typechecking

tests/helpers: restore symlink fallback on Windows if no administrative privileges or developer mode

With this change users no longer need to configure source url
explicitly via `repositories` configuration values when credentials are
required.

This change makes use of existing repository authentication mechanisms
to enable http authentication for git dependencies.

This change allows users to make use of find-links style single page
sources that do not present a PEP 503 simple API repository.

Co-authored-by: Agni Sairent <agniczech@gmail.com>
Co-authored-by: Dos Moonen <darsstar@gmail.com>

This change refactors HTTP repository source implementations. The
following changes have been made.

- CacheControl cache now lives within Authenticator.
- Authenticator manages unique sessions for individual netloc.
- CacheControl usage now respects disable cache parameter in repos.
- Certificate and authentication logic is now managed solely within
  Authenticator for source repositories taking advantage of recent
  enhancements.

These changes should allow for better handling of cases like those
described in #3041. Additionally, this forms the foundation for
unifying HTTP specific logic within the code base and possibly allowing
for migration of requests etc. if/when required.

With this change, users can now add dependencies using valid PEP 508
strings.

This change moves, cleans up and refactors dependency specification
parsing logic from `InitCommand` to
`poetry.utils.dependency_specification`. This is done to improve
usability and maintainability of this logic.

This change ensures that generated toml file from packages contain all
relevant metadata and handles groups correct.

* enable strict mypy checking

* update contribution guide

* remove mypy precommit hook

This change migrate's Poetry's homegrown appdirs implementation in 
favour of using the funtionality provided by the platformdirs package.

In order to support a breaking change introduced in platformdirs, we 
reuse existing user config directories on darwin platforms.

This makes the `virtualenv.options.no-pip` and
`virtualenv.options.no-setuptools` options explicitly available to
users.

We default these to false as otherwise it can cause breakages for
certain development tools.

clean up errors revealed by having a py.typed in poetry-core and running mypy in a proper environment

* more typechecking

* remove remaining `FooTypes` type annotations

just use the appropriate base class, no need for the extra complexity

* typecheck a couple more files

* more typechecking fixes

* more typechecking

* more typechecking

* tweak typing to appease pre-commit

* further mixology fixes

* consistency in repository package-finding

signal failure with `PackageNotFound`, rather than by returning None

* typechecking vcs.git.backend

updates:
- [github.com/pre-commit/mirrors-mypy: v0.942 → v0.950](https://github.com/pre-commit/mirrors-mypy/compare/v0.942...v0.950)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

This change introduces the config option
`experimental.system-git-client` defaulting to `false`. When set to
`true`, the subprocess git client implementation is used when cloning a
remote repository.

This option will be removed in a future release.

This change introduces dulwich as the git backend, instead of system
git executable. Together with an LRU cache when inspecting git package,
this considerable improves performance for dependency solver and reuse
of source when project has git dependencies.

In cases where dulwich fails with an HTTPUnauthorized error, Poetry
falls back to system provided git client as a temporary measure. This
will be replaced in the future once dulwich supports git credentials.

This change renames `safe_rmtree` into a more appropriate name with
relevant arguments. We ensure that the force behaviour is opt-in
and not the default to avoid ambiguity.

Relates-to: #5017