These changes make the Hy REPL more closely follow `code.InteractiveConsole`'s
class interface and provide minimally intrusive traceback print-out filtering
via a context manager that temporarily alters `sys.excepthook`. In other words,
exception messages from the REPL will no longer show Hy internal
code (e.g. importer, compiler and parsing functions).
The boolean variable `hy.errors._hy_filter_internal_errors` dynamically
enables/disables trace filtering, and the env variable
`HY_FILTER_INTERNAL_ERRORS` can be used as the initial value.
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