Dunamai

* Added `branch` and `timestamp` to the `Version` class,
along with associated format placeholders (`branch`, `branch_escaped`, `timestamp`).
Branch info is not populated for Darcs and Subversion repositories.
* Fixed validation for PEP 440, where the local segment was allowed to contain any characters.
* Fixed validation for Semantic Versioning, where some segments were allowed to contain
these additional characters:


[ \ ] ^ _ `

* Changed `Version.serialize`'s `format` argument to support passing a callback.
([Contributed by marnikow](https://github.com/mtkennerly/dunamai/pull/40))
* Added `ignore` option to `get_version()`.
([Contributed by marnikow](https://github.com/mtkennerly/dunamai/pull/39))
* Added `parser` option to `get_version()`.
* Added `Version.parse()`.
([Contributed by marnikow](https://github.com/mtkennerly/dunamai/pull/41))
* Added `Version.bump()`.
([Contributed by marnikow](https://github.com/mtkennerly/dunamai/pull/38))

* Changed the build backend to poetry-core.
([Contributed by fabaff](https://github.com/mtkennerly/dunamai/pull/35))
* Clarified serialization options that are ignored when using a custom format.
* Relaxed dependency range of `importlib-metadata` for compatibility with Poetry.
* Added `epoch` to `Version` class, default tag pattern, and format placeholders.
* Fixed PEP 440 validation to allow multiple digits in the epoch.
* Improved parsing of optional pattern groups so that we don't stop checking at
the first one that's omitted.
* Fixed handling of tags with `post`/`dev` stages so that they are serialized
and bumped correctly when using PEP 440.

* Broadened the default version tag pattern to allow more separator styles
recognized in PEP 440 pre-normalized forms (`-`, `.`, and `_`).
* Enhanced `serialize_pep440()` to normalize the alternative prerelease names
(`alpha` -> `a`, `beta` -> `b`, `c`/`pre`/`preview` -> `rc`) and
capitalizations (`RC` -> `rc`, etc).
* Added a `py.typed` file for PEP-561.
([Contributed by wwuck](https://github.com/mtkennerly/dunamai/pull/25))
* Replaced `pkg_resources` dependency with `packaging` and `importlib_metadata`.
([Contributed by flying-sheep](https://github.com/mtkennerly/dunamai/pull/29))
* Added some missing public items to `__all__`.

* Fixed an oversight where the default version tag pattern would only find
tags with exactly three parts in the base (e.g., `v1.0.0` and `v1.2.3`).
This is now relaxed so that `v1`, `v1.2.3.4`, and so on are also recognized.
* Added support for execution via `python -m dunamai`.
([Contributed by jstriebel](https://github.com/mtkennerly/dunamai/pull/19))

* Fixed handling of Git tags that contain slashes.
([Contributed by ioben](https://github.com/mtkennerly/dunamai/pull/17))