[PBCKP-357] fix UnicodeDecodeError

Got in some tests:
```
Traceback (most recent call last):
  File "pg_probackup/tests/replica.py", line 625, in test_replica_stop_lsn_null_offset
    gdb_checkpointer = self.gdb_attach(bgwriter_pid)
  File "pg_probackup/tests/helpers/ptrack_helpers.py", line 1984, in gdb_attach
    return GDBobj([str(pid)], self, attach=True)
  File "pg_probackup/tests/helpers/ptrack_helpers.py", line 2054, in __init__
    line = self.get_line()
  File "pg_probackup/tests/helpers/ptrack_helpers.py", line 2065, in get_line
    line = self.proc.stdout.readline()
  File "/usr/lib/python3.10/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xd0 in position 189: invalid continuation byte
```
Fixed with `errors='replace'`

Author: funny-falcon

tests/helpers/ptrack_helpers.py

@@ -2045,7 +2045,8 @@ def __init__(self, cmd, env, attach=False):
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             bufsize=0,
-            universal_newlines=True
+            text=True,
+            errors='replace',
         )
         self.gdb_pid = self.proc.pid