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
diff --git a/contrib/spoa_server/README b/contrib/spoa_server/README
index 341f5f9..f654a23 100644
--- a/contrib/spoa_server/README
+++ b/contrib/spoa_server/README
@@ -14,9 +14,10 @@
 You have to install the development packages, either from the
 distribution repositories or from the source.
 
-CentOS/RHEL: yum install python-devel
+CentOS/RHEL: sudo yum install python3-devel
 
-The current python version in use is 2.7.
+The current minimal python version compatible with this library is 2.7.
+It's recommended to use python version 3 where possible due to python 2 deprecation.
 
 
   Compilation
@@ -28,6 +29,11 @@
 You can add LUA_INC=.. LUA_LIB=.. to the make command to set the paths to
 the lua header files and lua libraries.
 
+Similarly, you can add PYTHON_INC=.. PYTHON_LIB=.. to the make command to set the paths to
+the python header files and python libraries.
+By default, it will try to compile by detecting the default python 3 parameters.
+It will fall back to python 2 if python 3 is not available.
+
   Start the service
 ---------------------