Add pyproject.toml
Now that pip 19 is out, we can attempt to re-add pyproject.toml

---

The release of pip version 10 and the presence of a `pyproject.toml` in our latest released versions gives problems for installing pandas in the following cases:

**Problem with installing pandas for Python 3.4**

* You get the error *"Double requirement given: numpy ..."*
* The reason for this error is that pandas 0.21-0.22 no longer supports Python 3.4, and thus no longer distributes binary wheels for Python 3.4. Therefore, pip tries to install pandas from source, and this is what now started to fail with Pip version 10.
* **Solution**:
  * Install an older version of pandas

    ```
    pip install pandas<0.21
    ```

  * Workaround for package maintainers to specify the pandas version in `install_requires`: https://github.com/pandas-dev/pandas/issues/20697#issuecomment-381372004
* The fact that those version are downloaded for Python 3.4 (instead of an older compatible version) was a bug in our setup.py, and this will be fixed in future versions of pandas: https://github.com/pandas-dev/pandas/pull/20698

**Problem with installing pandas on "special" platforms (32bit, PyPy, ARM, ...) (building from source)**:

* You get the error *"No matching distribution found for Cython"*
* The reason for this error is that we do not provide wheels for such python versions or platforms. In such a case, `pip` will try to build pandas from source, but this started to fail with pip version 10. This is because their limited support for PEP518, and they require that all build dependencies of pandas are installed from wheels (https://github.com/pypa/pip/issues/5229), which are often also not available on those python versions / platforms.
* **Solution**
  * Use an older version of pip
  * Use `pip install pandas --no-build-isolation`
* See eg https://github.com/pandas-dev/pandas/issues/20666, https://github.com/pandas-dev/pandas/issues/20771
