Mini-lambda

- packaging improvements: set the "universal wheel" flag to 1, and cleaned up the `setup.py`. In particular removed dependency to `six` for setup. Fixes [21](https://github.com/smarie/python-mini-lambda/issues/21)

- Added `__version__` attribute. Fixes [20](https://github.com/smarie/python-mini-lambda/issues/20)
- Improved `setup.py`, in particular long description now uses markdown (no pandoc anymore) and zip_safe is false. Fixes [18](https://github.com/smarie/python-mini-lambda/issues/18)
- Added `py.typed` file for PEP561 compliance. Fixes [19](https://github.com/smarie/python-mini-lambda/issues/19)

Added `pyproject.toml`.

When converting an expression into a function, the resulting callable object now has the same signature than inner `evaluate`. Fixes [17](https://github.com/smarie/python-mini-lambda/issues/17).

2.1.0 - New features to improve usability.

* Added two helper functions `is_mini_lambda_expr` and `as_function`. Fixes [13](https://github.com/smarie/python-mini-lambda/issues/13)

* Renamed `_LambdaExpression` to `LambdaExpression`. It is now exported at package lavel, that's clearer.

* Added `__repr__` to `LambdaFunction`. Fixes [14](https://github.com/smarie/python-mini-lambda/issues/14).

* in 2.0.0 pandas and numpy were mandatory again. Fixed that.

2.0.0 - python 2 support, default `repr`, and cleaner submodules structure

* Lambda expressions now have a normal `repr` by default, and this can be disabled by using the `repr_on` attribute to `False`. Fixes [12](https://github.com/smarie/python-mini-lambda/issues/12)

* Added support for python 2. Fixes [11](https://github.com/smarie/python-mini-lambda/issues/11).

* The package structure is now cleaner. In particular, predefined variables are in `mini_lambda.vars` and predefined symbols (constants, functions and classes) are in `mini_lambda.symbols`.

- Fixed [5](https://github.com/smarie/python-mini-lambda/issues/5) by making `numpy` and `pandas` import optional: they are now only imported if you import `mini_lambda.numpy_` or `mini_lambda.pandas_` respectively.

- New `And()` and `Or()` functions, see [7](https://github.com/smarie/python-mini-lambda/issues/7)