Convtools

**Features**

- added `__pow__` operator and corresponding `c.this.pow(...)` method
- added `c.expect(condition, error_msg=None)` and `c.this.expect(...)` method
which check input for condition and raises `c.ExpectException` if condition
fails

**Deprecations**

- The following reducers never really worked with `initial` keyword argument
because the underlying data structure was never exposed and this led to
undefined behavior. Until `v2.0` such reducers with `internals_are_public ==
False` will ignore `initial`, in `v2.0` if `initial` is passed, they will
raise `ValueError`:


* ArrayDistinct
* CountDistinct
* First
* Last
* MaxRow
* MinRow
* SortedArray
* Dict
* DictArray
* DictArrayDistinct
* DictCount
* DictCountDistinct
* DictFirst
* DictLast
* DictMax
* DictMin
* DictSum
* DictSumOrNone


**Bugfix**

- now `c.ReduceFuncs.Count()` counts rows, while `c.ReduceFuncs.Count(c.this)`
counts not `None` values; both of these used to calculate rows (like SQL's
`count(*)`)
- now `c.ReduceFuncs.DictCount(c.this)` counts rows, while
`c.ReduceFuncs.DictCount(c.item("key"), c.item("value"))` counts not `None`
values; both of these used to calculate rows (*like SQL's `count(*)`*)

**Bugfix**

- fixed `c.naive` internals to work with `pypy`

**Misc**

- weakened python version requirements: changed `">=3.6,<3.12"` to `">=3.6"`

**Features**

- added `default` to `c.date_parse` and `c.datetime_parse` to return when no
formats fit

- 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`