This commit mildly refactors the codebase for forward-compatibility with
PySide2 >= 5.14.0, rendering the codebase unavoidably incompatible with
prior PySide2 versions. Specifically, this commit:

* Dependencies bumped:
  * PySide2 >= 5.14.0, which removed the requisite "pyside2-uic" and
    "pyside2-rcc" commands in favour of the "uic --generate python" and
    "rcc --generate python" commands provided by both the optional
    "pyside2-tools" dependency and mandatory C++ Qt dependencies. Ergo,
    "pyside2-tools" is mostly no longer required at runtime.
* API generalized:
  * Generalized the "betse.lib.pyside2.cache.guipsdcacheqrc" submodule:
    * Generalized the _cache_py_qrc_file() cacher to reference the
      requisite "rcc" rather than obsolete "pyside2-rcc" command.
  * Generalized the "betse.lib.pyside2.cache.guipsdcacheqrc" submodule:
    * Generalized the convert_qrc_to_py_file() converter to call the
      requisite "rcc" rather than obsolete "pyside2-rcc" command with
      requisite Python 3.x parameters (e.g., "--generate python").

Unrelatedly, this commit also bumps all copyright notices embedded
throughout the codebase. (Unitarian utilitarianism militates illiterate totalitarianism!)