Fix scalar values in SNMP set requests

[SoundGoof]

Summary

Fixes #6 by allowing SNMP SET requests to accept canonical Python scalar values instead of requiring every value to be a bytes object.

The native extension now:

• Preserves bytes values without modification, including binary OCTET STRING data.
• Converts integers, strings, and other non-byte values using their canonical str() representation.
• Keeps the Python object containing the converted value alive while constructing the SNMP PDU.
• Propagates conversion errors directly instead of returning a secondary SystemError.

Root Cause

Varbind.val preserves its original Python type, but the SET implementation passed every value directly to PyBytes_AsStringAndSize().

This worked for bytes, but rejected integers, Python strings, and other scalar values before the SNMP request could be constructed.

The conversion helper now distinguishes between byte and non-byte values:

• bytes values are passed through unchanged.
• Other values are converted with str() and encoded through Python's UTF-8 representation.
• The resulting Python object remains referenced until Net-SNMP has copied the value into the PDU.

Testing

The system tests run against a private Ubuntu snmpd instance.

Coverage includes:

• Real SET→GET round trips for INTEGER values supplied as Python integers, strings, and byte strings.
• Real SET→GET round trips for textual OCTET STRING values supplied as Python strings and byte strings.
• A writable pass_persist INTEGER scalar, used because Ubuntu's packaged agent does not expose a suitable writable INTEGER object.
• Packet-level verification that binary OCTET STRING values containing embedded NUL and non-UTF-8 bytes are preserved in the emitted BER payload.
• Existing SNMPv1, SNMPv2c, and SNMPv3 system tests.

Results:

• All 45 system tests pass normally.
• All 45 system tests pass under Valgrind.
• Valgrind reports 0 errors.
• Valgrind reports no definitely, indirectly, or possibly lost memory.