Convtools

**Misc:**

- stabilized API

**BREAKING CHANGES:**

- renamed all non-public modules so the only supported way to import is
directly from "convtools", e.g.:

* `from convtools import conversion as c`
* `from convtools import DateGrid, DateTimeGrid`

- contrib ones are left as is:

* `from convtools.contrib.tables import Table`
* `from convtools.contrib.fs import split_buffer`

**Misc**

- locked CI dependencies
- switched packaging to hatch
- dropped setup.py and setup.cfg in favor of pyproject.toml

**Bugfix**

- fixed `c.group_by` for cases with input_arg/label inside and
iter/as_type/sort/tap methods called on it

**Bugfix**

- fixed `c.group_by` for cases where group by keys are generated functions
(e.g. deep attr lookup with default). No worries about silent errors, it used
to fail hard.

**Bugfix**

- fixed `Table.join` for cases where the right part has internal row type
different from the left one

**Features**

- introduced `c.iter_unique(element_conv=None, by_=None)` and
`(...).iter_unique` methods which define a conversion, which iterates over
the input and yields values unique based on the provided condition