Switch package to using Python 3

This MR is a first effort at using Python 3 for adequate. It changes the packaging and the test suite to use Python 3, updating the paths where necessary and also dropping a Python 2-specific test of pyshared.

Note that there are two additional library symbol tests in the test suite that currently fail; these failures are unrelated to this MR.

install adequate-testpkg-missing-symbol-version-information ... ok
check adequate-testpkg-missing-symbol-version-information ... ERROR (non-empty stderr)
! adequate: ldd -r /usr/bin/adequate-test-msvi: Inconsistency detected by ld.so: dl-lookup.c: 111: check_match: Assertion `version->filename == NULL || ! _dl_name_match_p (version->filename, map)' failed!

install adequate-testpkg-undefined-symbol ... ok
check adequate-testpkg-undefined-symbol ... FAIL
-adequate-testpkg-undefined-symbol: undefined-symbol /usr/bin/adequate-test-us1 => this_symbol_might_be_undefined
-adequate-testpkg-undefined-symbol: undefined-symbol /usr/bin/adequate-test-us2 => this_symbol_might_be_undefined@ADEQUATE_TEST (libadequate-test-versioning.so.0)
+adequate-testpkg-undefined-symbol: undefined-symbol /usr/bin/adequate-test-us2 => this_symbol_might_be_undefined@ADEQUATE_TEST`