Bump to 0.12.15

jsc_custom/_version.py

 # __version__ should be updated using tbump, based on configuration in
 # pyproject.toml, according to instructions in RELEASE.md.
 #
-__version__ = "0.12.14"
+__version__ = "0.12.15"

pyproject.toml

@@ -56,7 +56,7 @@ profile = "black"
 github_url = "https://github.com/kreuzert/jupyter-jsc-custom"
 
 [tool.tbump.version]
-current = "0.12.14"
+current = "0.12.15"
 regex = '''
   (?P<major>\d+)
   \.