Passing Solver explicitly causes UnboundLocalError

**Describe the bug**

I accidentally wrote the following (incorrect - the parameter should be `solver_name`) code:

```
import mip

model = mip.Model(sense=mip.MINIMIZE, solver=mip.CBC)
```

This fails with the following rather surprising error:

```
Traceback (most recent call last):
File "/Users/drmaciver/Projects/sat-sampling/check_mip.py", line 3, in
model = mip.Model(sense=mip.MINIMIZE, solver=mip.CBC)
File "/Users/drmaciver/Projects/sat-sampling/.venv/lib/python3.10/site-packages/mip/model.py", line 104, in __init__
self.constrs = mip.ConstrList(self)
UnboundLocalError: local variable 'mip' referenced before assignment
```

The reason for this is that the `if not solver` path in model.py's `Model.__init__` includes various imports that look like e.g. `import mip.gurobi`. Because of the way Python imports work, this means that `mip` is now a local variable shadowing the global `import mip`. In paths where that import statement has not been run (i.e. when a solver argument is explicitly passed) attempts to use `mip` results in this `UnboundLocalError`

**Expected behavior**

As mentioned this is incorrect code, so it's reasonable to raise an error here, although the specific error raised is quite confusing (probably a TypeError would be appropriate, but if I'd been running a static checker it would have caught it).

The more important problem is that correct code using this parameter would also fail in an identical way - currently the `solver` parameter cannot be used at all, because any attempt to use it will trigger this bug.

This should be an easy enough fix (adding an unconditional `import mip` after the if block will do it for example).

**Desktop (please complete the following information):**

Tested on OSX, 3.10.11 (this shouldn't matter), with mip 1.15.0 (latest version right now).