Bad serialization of anti-slash in regexps with /.../ syntax

Using version 1.2.0

```python
import jsonpath

jp = jsonpath.compile(r"$[?@ =~ /\d/]")
assert jp.findall(["", "1", "10"]) == ["1"]  # OK

assert str(jp) == r"$[?@ =~ /\d/]"  # NOK: "\d" --> "d"

jp_from_str = jsonpath.compile(str(jp))
assert jp.findall(["", "1", "10"]) == ["1"]  # NOK
```

Note that behavior is OK with `r"$[?match(@, '\\d')]"` (but anti-slash needs to be escaped).