pgml is not compatible with plpython, if using both pgml and plpython in the
same session, postgresql will crash.

minimum reproducible code:

```sql
SELECT pgml.embed('intfloat/e5-small', 'hi mom');

create or replace function pyudf()
returns int as
$$
return 0
$$ language 'plpython3u';
```

the call stack:

```
 Stack trace of thread 161970:
 #0  0x00007efc1429edb8 PyImport_Import (libpython3.9.so.1.0 + 0x9edb8)
 postgresml#1  0x00007efc1429f125 PyImport_ImportModule (libpython3.9.so.1.0 + 0x9f125)
 postgresml#2  0x00007efb04b0f496 n/a (plpython3.so + 0x10496)
 postgresml#3  0x00007efb04b1039d plpython3_validator (plpython3.so + 0x1139d)
 postgresml#4  0x0000559d0cdbc5c2 OidFunctionCall1Coll (postgres + 0x6465c2)
 postgresml#5  0x0000559d0c9d68bb ProcedureCreate (postgres + 0x2608bb)
 postgresml#6  0x0000559d0ca5030c CreateFunction (postgres + 0x2da30c)
 postgresml#7  0x0000559d0ce1c730 n/a (postgres + 0x6a6730)
 postgresml#8  0x0000559d0cc5a030 standard_ProcessUtility (postgres + 0x4e4030)
 postgresml#9  0x0000559d0cc545ed n/a (postgres + 0x4de5ed)
 postgresml#10 0x0000559d0cc546e7 n/a (postgres + 0x4de6e7)
 postgresml#11 0x0000559d0cc54beb PortalRun (postgres + 0x4debeb)
 postgresml#12 0x0000559d0cc55249 n/a (postgres + 0x4df249)
 postgresml#13 0x0000559d0cc576f0 PostgresMain (postgres + 0x4e16f0)
 postgresml#14 0x0000559d0cbc3e9c n/a (postgres + 0x44de9c)
 postgresml#15 0x0000559d0cbc50aa PostmasterMain (postgres + 0x44f0aa)
 postgresml#16 0x0000559d0c8ce7d2 main (postgres + 0x1587d2)
 postgresml#17 0x00007efc18427cd0 n/a (libc.so.6 + 0x27cd0)
 postgresml#18 0x00007efc18427d8a __libc_start_main (libc.so.6 + 0x27d8a)
 postgresml#19 0x0000559d0c8cee15 _start (postgres + 0x158e15)
```

this is because PostgreSQL is using dlopen(RTLD_GLOBAL). this will parse some
of symbols into the previous opened .so file, but the others will use a
relative offset in pgml.so, and will cause a null-pointer crash.

this commit hide all symbols except the UDF symbols (ends with `_wrapper`) and
the magic symbols (`_PG_init` `Pg_magic_func`). so dlopen(RTLD_GLOBAL) will
parse the symbols to the correct position.

Cleanup installation instructions