check_slow_queries_digest_result: fix encoding on file opening

Sometimes, it seems the file analyzed can contain non-ASCII characters, giving
errors such as:

```
codec can't decode byte 0xc3 in position 5138: ordinal not in range(128)
```

It seems there is no reason to not support UTF8, and more generally ignore
encoding errors here. This change has been applied for a few weeks on Nexedi
ERP5 production without problems.

/reviewed-by @tomo @jerome
/reviewed-on !140
/cc @xavier_thompson