Added ``astropy.config.temporary_cache_dir_path`` and
``astropy.config.temporary_config_dir_path`` context managers, which are
safer alternatives to ``astropy.config.set_temp_cache`` and
``astropy.config.set_temp_config`` respectively, and should be preferred in
new code, but are not drop-in replacements.

Added support for ``ASTROPY_CACHE_DIR`` and ``ASTROPY_CONFIG_DIR``
environment variables, offering a more tightly scoped alternative to
``XDG_CACHE_HOME`` and ``XDG_CONFIG_HOME`` respectively.
When both are defined, ``ASTROPY_`` -prefixed variables take precedence
over ``XDG_`` -prefixed ones.
