Update version regex according to latest packaging version

According to `pypa/packaging#705`.

Author: abravalheri

src/validate_pyproject/formats.py

@@ -21,7 +21,7 @@
         (?P<release>[0-9]+(?:\.[0-9]+)*)                  # release segment
         (?P<pre>                                          # pre-release
             [-_\.]?
-            (?P<pre_l>(a|b|c|rc|alpha|beta|pre|preview))
+            (?P<pre_l>alpha|a|beta|b|preview|pre|c|rc)
             [-_\.]?
             (?P<pre_n>[0-9]+)?
         )?