MAJOR: contrib: porting spoa_server to support python3

MAJOR: contrib: porting spoa_server to support python3

Background:
    Python 2 is no longer supported since January, 1st 2020 as per
    https://www.python.org/doc/sunset-python-2/
    The purpose of this change is to make the spoa_server contrib library
    compatible with Python 3 to allow transition to Python 3.

Test Settings:
ps_python.py:
    ...
    spoa.set_var_null("null", spoa.scope_txn)
    spoa.set_var_boolean("boolean", spoa.scope_txn, True)
    spoa.set_var_int32("int32", spoa.scope_txn, 1234)
    spoa.set_var_uint32("uint32", spoa.scope_txn, 1234)
    spoa.set_var_int64("int64", spoa.scope_txn, 1234)
    spoa.set_var_uint64("uint64", spoa.scope_txn, 1234)
    spoa.set_var_ipv4("ipv4", spoa.scope_txn, ipaddress.IPv4Address(u"127.0.0.1"))
    spoa.set_var_ipv6("ipv6", spoa.scope_txn, ipaddress.IPv6Address(u"1::f"))
    spoa.set_var_str("str", spoa.scope_txn, "1::f")
    spoa.set_var_bin("bin", spoa.scope_txn, "1:\x01:\x42f\x63\x63")
    spoa.set_var_str("python_version", spoa.scope_sess, str(sys.version_info))
    ...
haproxy.cfg:
    ...
    http-request capture var(txn.verb.null),debug len 1
    http-request capture var(txn.verb.boolean),debug len 1
    http-request capture var(txn.verb.int32),debug len 4
    http-request capture var(txn.verb.uint32),debug len 4
    http-request capture var(txn.verb.int64),debug len 4
    http-request capture var(txn.verb.uint64),debug len 4
    http-request capture var(txn.verb.ipv4),debug len 16
    http-request capture var(txn.verb.ipv6),debug len 45
    http-request capture var(txn.verb.str),debug len 32
    http-request capture var(txn.verb.bin),debug len 32
    http-request capture var(sess.verb.python_version),debug len 100
    ...

Test result:
    Python 3.8:
        ft_public ft_public/<NOSRV> 0/-1/-1/-1/0 403 212 - - PR-- 1/1/0/0/0 0/0 {|1|1234|1234|1234|1234|127.0.0.1|1::f|1::f|1:#01:Bfcc|sys.version_info(major=3, minor=8, micro=1, releaselevel='final', serial=0)} "POST / HTTP/1.1"
    Python 3.7:
        ft_public ft_public/<NOSRV> 0/-1/-1/-1/0 403 212 - - PR-- 1/1/0/0/0 0/0 {|1|1234|1234|1234|1234|127.0.0.1|1::f|1::f|1:#01:Bfcc|sys.version_info(major=3, minor=7, micro=6, releaselevel='final', serial=0)} "POST / HTTP/1.1"
    Python 3.6:
        ft_public ft_public/<NOSRV> 0/-1/-1/-1/0 403 212 - - PR-- 1/1/0/0/0 0/0 {|1|1234|1234|1234|1234|127.0.0.1|1::f|1::f|1:#01:Bfcc|sys.version_info(major=3, minor=6, micro=10, releaselevel='final', serial=0)} "POST / HTTP/1.1"
    Python 2.7:
        ft_public ft_public/<NOSRV> 0/-1/-1/-1/0 403 212 - - PR-- 1/1/0/0/0 0/0 {|1|1234|1234|1234|1234|127.0.0.1|1::f|1::f|1:#01:Bfcc|sys.version_info(major=2, minor=7, micro=17, releaselevel='final', serial=0)} "POST / HTTP/1.1"

Not tested:
Python <2.7