Make the version detection slightly more flexible

Just in case someone using GPax as a dev copy does not have dunamai
installed, GPax will now gracefully fail to detect the version
and will simply use "dev" as the version name.

gpax/_version.py

@@ -1,7 +1,10 @@
 """Version file. This is overwritten during build and will contain a static
 __version__ variable."""
 
-import dunamai as _dunamai
+try:
+    import dunamai as _dunamai
 
-__version__ = _dunamai.Version.from_any_vcs().serialize()
-del _dunamai
+    __version__ = _dunamai.Version.from_any_vcs().serialize()
+    del _dunamai
+except ImportError:
+    __version__ = "dev"

scripts/build_project.sh

@@ -1,5 +1,6 @@
 #!/bin/bash
 
 pip install flit~=3.7
+pip install dunamai==1.19.2
 echo "__version__ = '$(dunamai from any --style=pep440 --no-metadata)'" >gpax/_version.py
 flit build