False positive check inserting inserting identities with l and ł values

Values with polish characters `l` and `ł` are considered equal in the database giving false positives when a similar identities with these characters are inserted. For example, having the next identity in the database:

```
AAAAAAAA Michał Smith michalsmith@example.com msmith unknown
```

The call to the next command fails when the name is written with latin characters:
```
sortinghat add --name "Michal Smith" --email "michalsmith@example.com" --username "msmith"
Error: unique identity 'Michal Smith-michalsmith@example.com-msmith-unknown' already exists in the registry

Start by reproducing the issue with the `sortinghat add` command using `Michal Smith` and `Michał Smith`, and inspect how identity uniqueness is checked against the database. Done means identities differing by `l` and `ł` are handled distinctly without the reported false-positive error, with the behavior covered by a regression test.

Written by the indexing model from the issue text.