* fix non-python packages passed via packagesExtra
* refactor examples and readme for 3.0.0
* add changelog for 3.0.0
* bump version to 3.0.0 and update nixpkgs and pypi-deps-db dependencies
# 2.2.2 (17 Aug 2020)
### Fixes
- Packages with dot in name led to invalid nix expression
- Problem generating error message for resolution impossible errors
- `buildPythonPackage` of mach-nix failed if arguments like `pkgs` were passed.
- When overriding packages, mach-nix now falls back to using `overrideAttrs` if `overridePythonAttrs` is not available.
### Package Fixes:
- pip: installation failed. Fixed by forcing `nixpkgs` provider
- gdal: building from sdist doesn't work. Fixed by forcing `nixpkgs` provider
### Development
- Merged project `pypi-crawlers` into mach-nix (was separated project before)
# 2.2.0 (09 Aug 2020)
Improved success rate, MacOS support, bugfixes, optimizations
### Features
- Improved selection of wheel releases. MacOS is now supported and architectures besides x86_64 should be handled correctly.
- Whenever mach-nix resolves dependencies, a visualization of the resulting dependency tree is printed on the terminal.
- The dependency DB is now accessed through a caching layer which reduces the resolver's CPU time significantly for larger environments.
- The python platform context is now generated from the nix build environment variable `system`. This should decrease the chance of impurities during dependency resolution.
### Fixes
- The requires_python attribute of wheels was not respected. This lead to failing builds especially for older python versions. Now `requires_python` is part of the dependency graph and affects resolution.
- Detecting the correct package name for python packages in nixpkgs often failed since the attribute names don't follow a fixed schema. This lead to a handful of different errors in different situations. Now the package names are extracted from the pypi `url` inside the `src` attribute which is much more reliable. For packages which are not fetched from pypi, the `pname` attribute is used as fallback.
- Fixed bug which lead to the error `attribute 'sdist' missing` if a package from the nixpkgs provider was used which doesn't publish it's source on pypi. (For example `tensorflow`)
### Other Changes
- Mach-nix now uses a revision of the nixpkgs-unstable branch instead of nixos-20.03 as base fo the tool and the nixpkgs provider.
- Updated revision of the dependency DB
