fix: Gracefully handle read-only sdists

For a package without dependencies in pypi's database, like p4python,
the sdist is required.  The problem is p4python was developed in
perforce where all files are read-only by default and deleting the temp
directory fails.

So we need to use the custom-built temp directory and specially handle
read-only files to be able to use p4python in poetry.

Fixes #520

Bump actions/cache from v1 to v2

Small improvements to docs for version command

Prior to this fix, when formatting PEP 508 dependency specification for
remote url references, the quoted marker was not prefixed with required
whitespace. This change ensures that the format adheres to the 
specified grammar.

Resolves: python-poetry/poetry#2326

Fix typo in docs

Your -> you

* fix incorrect parsing of spaces when parsing version constraints
* add tests for `parse_constraint`

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

Co-Authored-By: Pradyun Gedam <pradyunsg@gmail.com>

Co-Authored-By: Pradyun Gedam <pradyunsg@gmail.com>

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

Relates-to: #2238

Relates-to: #2238

Resolves: #2115

'An' should come before a vowel sound.

Reference: https://english.stackexchange.com/a/164/198117

When testing the behavior for git packages with a setup.py file, the Provider class would create a temporary virtual environment and execute python setup.py egg_info. Both actions are costly and take time. By mocking them, we can reduce the tests time by a factor of 4 or 5.

Co-authored-by: finswimmer77@gmail.com <GiWahnsinn07!>

* Add fish shell tab completion for mac

* Remove macOS ref as brew should work linux as well

If multiple distributions have `tests` as a top-level package, they'll conflict whenever both are installed. (Examples [here]((https://github.com/python-poetry/poetry/issues/1905) and [here](https://github.com/NixOS/nixpkgs/issues/81482). Two common alternative strategies are:

1. not distributing tests (as [here](https://github.com/pypa/sampleproject)), or 
2. placing tests in a subdirectory of the main package, rather than adjacent (as [here](http://blog.habnab.it/blog/2013/07/21/python-packages-and-you/) and [here](http://as.ynchrono.us/2007/12/filesystem-structure-of-python-project_21.html)). Each of these strategies will avoid this issue. 

Users may fall into this trap because of the [package documentation](https://python-poetry.org/docs/pyproject/#packages) page, which gives an example:

```
packages = [
    { include = "my_package" },
    { include = "tests", format = "sdist" },
]
```

Having two top-level packages in a distribution is relatively unusual, but does have some use cases.  Using `"tests"` as a top-level package name in the example is likely to lead to conflicts, however. The alternate package in the documentation example could have a unique name like `"my_other_package"`, which would reduce the likelihood of this kind of overlap.

Resolves: #2198

Configuring the template chooser to include discord community server.