Fix Python warning about invalid escape sequences

[mkoncek]

Currently, when running any Java package package builds in Fedora Rawhide, we can see this warning:

...
+ /usr/bin/python3 /usr/share/java-utils/mvn_build.py --xmvn-javadoc -j
/usr/lib/python3.14/site-packages/javapackages/maven/artifact.py:194: SyntaxWarning: "\{" is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\{"? A raw string is also an option.
  prog = re.compile("\\$\{([^}]+)\}")
...