fix metadata arglists for python defaults

[jsavyasachi]

Fixes #284.
Fixes #283.

Changes

• Preserve values returned in argspec defaults when generating pyarglists.
• Stringify Python-object defaults, such as <class 'int'>, instead of exposing pointer metadata.
• Add :arglists metadata for vars created by py/from-import, so Clojure doc output can render the signature before the docstring.
• Add regression coverage for positional defaults, keyword-only defaults, Python-object defaults, and rendered clojure.repl/doc output.

Verification

env PATH=/private/tmp/libpython-clj-x86-python-shim:$PATH clojure -M:test -n libpython-clj2.metadata-test
env PATH=/private/tmp/libpython-clj-x86-python-shim:$PATH clojure -M:test -n libpython-clj2.python-test
env PATH=/private/tmp/libpython-clj-x86-python-shim:$PATH clojure -M:test

Note: the PATH shim was needed locally because Java is x86_64 while the default Homebrew Python is arm64.