provider: ensure download dest file is a string

Resolves: #98

Before this change, when .venv directory exists  within the project 
root, poetry will use it as the path to the venv regardless whether 
virtualenvs.in-project is set to true or not. This leads to confusion
as described in #1770 and #2756.

With this change poetry will check if virtualenvs.in-project is set to
true if it finds a .venv folder.

This commit also changes the default state of this configuration to be
unset (null), in order to not break any current environments.

Resolves: #1770 #2756

* changelog: fix 1.1.0b4 links

* changelog: add missing change for 1.1.0b3

Resolves: #2931

Bump version to 1.1.0b4

* dep: update identify

* deps: update requests-toolbelt

Resolves: #2939

I also added the `-` argument to `python`, because without it you cannot simply append flags for the script.
Hope this is just the same for powershell, as for bash.

Cf. #898

Fix inconsistencies with release scripts

Python 2 running on Windows, causes thread leaks which
leads console io write errors when using pastel. This
works around the issue by limiting to a single worker
in these environments.

Release 1.1.0b3

Fix and improve handling of extras while resolving dependencies

* Improve performance of package ordering/tagging after version solving has been completed

* Fix CI errors

Co-authored-by: Zack Weger <zackary.weger@aon.com>

* Display actual repository name in publish, not always PyPI

FreeBSD build environments cannot use `get-poetry.py` script as this
requires a release to be bootstrapped. These environments also do not
support Python 3.9 yet. This change ensures that non-linux
environments use `pip` to install poetry.

Resolves: #2815

Resolves: #2376

This change ensures that PEP 517 fallback builds are enabled by default
for when metadata inspection and setup.{py,cfg} parsing fails. This is
required in order to ensure that package requirements are correctly
identified for packages beyond first level dependencies.

Resolves: #2807

This change ensures that packages that only have a pre-release versions
are pragmatically discovered via `find_packages` when a constraint (*)
is specified.

Resolves: #2819

The existing example:
```bash
poetry export -f requirements.txt > requirements.txt
```
The problem with this is that any additional output from poetry will also end up in the requirements file. For instance, if the `poetry.lock` file is not present, poetry will output to stdout some extra text telling you about this. This breaks the requirements file. (I was burned by this problem, hence this PR)

Seeing as there is already a documented option to specify the filename directly, which resolves this issue, updating the docs like this seems reasonable.

* Add note for setting up poetry in a pre-existing project.

Resolves: #2630

* Show poetry init in more detail

* Update docs to reflect real poetry init usage

This change ensures that the executor does not deadlock on IO errors,
for example when IO does not support non-ascii characters.