CockroachDB is not automatically setting server_version in all cases
This is now resolve, thanks for the report. In the meantime you can alternatively use the database itself as a context-manager, which ensures a connection is created and wraps the block in a transaction (assuming this is for module-scope type initialization or whatever), e.g.

```python
with db:
    ...
```
I am occasionally seeing the error message below that is not especially self-explanatory.

```
    158 def atomic(self, system_time=None, priority=None):
--> 159     if self.server_version < NESTED_TX_MIN_VERSION:
    160         return _crdb_atomic(self, system_time, priority)
    161     return super(CockroachDatabase, self).atomic(system_time, priority)

TypeError: '<' not supported between instances of 'NoneType' and 'int'
```

I am using CockroachDB. The `server_version` is set when a connection is opened, which `atomic()` normally does. But in line 159 of `cockroachdb.py`, it's assumed `server_version` has already been set before calling `super(...).atomic()`.

https://github.com/coleifer/peewee/blob/49792e4eac5268c752375d34bbb63828708b592b/playhouse/cockroachdb.py#L159

To reproduce the error:

```python
from playhouse.db_url import connect

db = connect("cockroachdb://username:password@gcp-us-east1.cockroachlabs.cloud:26257/defaultdb...")

with db.atomic():  # raises error
    pass
```

I do not see this error if I use the `PostgresqlDatabase` class (e.g., when I change the schema of the datatbase url only).

```python
from playhouse.db_url import connect

db = connect("postgresql://username:password@gcp-us-east1.cockroachlabs.cloud:26257/defaultdb...")  # same other than schema

with db.atomic():  # no error
    pass
```

I am using peewee version `3.14.10`.