Development towards 0.5.0

[AndrejOrsula]

• Bump pyo3 to 0.21
  • Updated to use pyo3::Bound smart pointers (both internal and generated code)
• Generate pyo3_embed_python_source_code() when using Codegen::module_from_str()
  • This simplifies the setup requested by How can I generate my own code instead of a third-party library? #21
• Remove macros from the default features
  • This attempts to avoid issues mentioned in Crate not found error #22
• Add numpy feature for automatic mapping of numpy::ndarray

Other significant API changes for the generated bindings

1. Calling methods of class MyClass(pyo3::PyAny) encapsulated as pyo3::Bound<'_, MyClass> is now a bit more cumbersome. It requires that use MyClassMethods; is included in the scope when such methods are used. This follows the API design of PyO3, where all methods are implemented under a trait (e.g. pyo3::types::PyAnyMethods) due to arbitrary_self_types being unstable.
2. Passing of ::pyo3::marker::Python<'py> is no longer necessary for methods. It is extracted from self directly.

[codecov]

Codecov Report

Attention: Patch coverage is 68.30835% with 148 lines in your changes are missing coverage. Please review.

Project coverage is 66.77%. Comparing base (6466a7f) to head (2494035).
Report is 1 commits behind head on main.

Files	Patch %	Lines
pyo3_bindgen_engine/src/typing/into_rs.rs	33.87%	82 Missing ⚠️
pyo3_bindgen_engine/src/syntax/module.rs	69.13%	25 Missing ⚠️
...en_engine/src/syntax/common/function_definition.rs	0.00%	12 Missing ⚠️
pyo3_bindgen_engine/src/codegen.rs	70.83%	7 Missing ⚠️
pyo3_bindgen_engine/src/syntax/function.rs	86.53%	7 Missing ⚠️
pyo3_bindgen_engine/src/syntax/property.rs	90.78%	7 Missing ⚠️
pyo3_bindgen_engine/src/typing/from_py.rs	73.33%	4 Missing ⚠️
pyo3_bindgen_engine/src/syntax/class.rs	95.23%	3 Missing ⚠️
pyo3_bindgen_macros/src/utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   67.45%   66.77%   -0.69%     
==========================================
  Files          21       22       +1     
  Lines        3008     3211     +203     
==========================================
+ Hits         2029     2144     +115     
- Misses        979     1067      +88     

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