add Poetry instead of setuptools

[ftapajos]

this pr substitutes setuptools for poetry and moves root snmp-server.py to the tested folder. This substitution yields the following failed tests, which concern me a bit:

======================================================= short test summary info ========================================================
FAILED snmp_server/tests/test_server.py::Test::test_counter32 - AssertionError: b'A\x02\x00\xff' != b'A\x01\xff'
FAILED snmp_server/tests/test_server.py::Test::test_counter64 - AssertionError: b'F\x02\x00\xff' != b'F\x01\xff'
FAILED snmp_server/tests/test_server.py::Test::test_gauge32 - AssertionError: b'B\x04\x00\x00\x00\x00' != b'B\x08\x00\x00\x00\x00\x00\x00\x00\x00'
FAILED snmp_server/tests/test_server.py::Test::test_integer - AssertionError: b'\x02\x04\x00\x00\x00\x00' != b'\x02\x08\x00\x00\x00\x00\x00\x00\x00\x00'
FAILED snmp_server/tests/test_server.py::Test::test_parse_asn1_length - AssertionError: 48225 != 12345678
FAILED snmp_server/tests/test_server.py::Test::test_timeticks - AssertionError: b'C\x02\x00\xff' != b'C\x01\xff'
FAILED snmp_server/tests/test_server.py::Test::test_write_asn1_length - AssertionError: b'\x81\x00\x80' != b'\x81\x80'
FAILED snmp_server/tests/test_server.py::Test::test_write_int - AssertionError: b'\x00\x80' != b'\x80'
FAILED snmp_server/tests/test_server.py::Test::test_write_tlv - AssertionError: b'\xff\x81\x00\x80value' != b'\xff\x81\x80value'
ERROR snmp_server/tests/test_plugin.py::test_request_replies_correctly
ERROR snmp_server/tests/test_plugin.py::test_dual_request_replies_correctly
ERROR snmp_server/tests/test_plugin.py::test_snmp_range_correctly
ERROR snmp_server/tests/test_plugin.py::test_snmp_walk
================================================ 9 failed, 19 passed, 4 errors in 0.10s ================================================

[delimitry]

Hi @ftapajos! Thank you!
I've found a problem, will fix it.