sagemathgh-41122: Subsets of primes defined by congruence conditions
    
We implement subsets of primes given by congruence conditions and
operations on those sets.

Here is a short demo.

```
sage: P = Primes(modulus=4); P
Set of prime numbers congruent to 1 modulo 4: 5, 13, 17, 29, ...
sage: P[:10]
[5, 13, 17, 29, 37, 41, 53, 61, 73, 89]
sage: P.next(500)
509
sage: for p in P:
....:     if p > 50: break
....:     print(p)
5
13
17
29
37
41
```

```
sage: P = Primes(modulus=4); P
Set of prime numbers congruent to 1 modulo 4: 5, 13, 17, 29, ...
sage: Q = Primes(modulus=4, classes=[3])
Set of prime numbers congruent to 3 modulo 4: 3, 7, 11, 19, ...
sage: PQ = P.union(Q)
sage: PQ
Set of all prime numbers with 2 excluded: 3, 5, 7, 11, ...
sage: PQ == Primes().exclude(2)
True
```

Cc @FloFuer

### 📝 Checklist

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- [x] The title is concise and informative.
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preview.
    
URL: sagemath#41122
Reported by: Xavier Caruso
Reviewer(s): Martin Rubey, Xavier Caruso

Author: Release Manager