fix: make local installation work again with pip 23.2 and above

Surprise, Python doesn't support `-SNAPSHOT` versions like Maven
does! Moreover, pip version 23.2 removed support for versions
which don't conform to PEP 440.

So we make the snapshot version conformant by using the `.dev`
suffix (equivalent to `.dev0`) which basically serves the same
purpose as `-SNAPSHOT`. By this, we make this snippet from the
README work again:

```sh
git clone https://github.com/miyuchina/mistletoe.git
cd mistletoe
pip3 install -e .
```

mistletoe/__init__.py

@@ -2,7 +2,7 @@
 Make mistletoe easier to import.
 """
 
-__version__ = "1.3.0-SNAPSHOT"
+__version__ = "1.3.0.dev"
 __all__ = ['html_renderer', 'ast_renderer', 'block_token', 'block_tokenizer',
            'span_token', 'span_tokenizer']