`DoltSQLServerContext` connection fails when there are unstaged changes

```
$ dolt status
On branch test_data_types
Untracked files:
(use "dolt add " to include in what will be committed)
new table: mytable
```

```python
with DoltSQLServerContext(dolt, ServerConfig(user="root", password="")) as doltsql:
df = doltsql.read_pandas("mytable")

ValueError Traceback (most recent call last)
Input In [8], in ()
----> 1 with DoltSQLServerContext(dolt, ServerConfig(user="root", password="")) as doltsql:
2 df = doltsql.read_pandas("mytable")

File ~/.cache/pypoetry/virtualenvs/science-s3M63NV5-py3.9/lib/python3.9/site-packages/doltpy/sql/sql.py:368, in DoltSQLServerContext.__enter__(self)
365 def __enter__(self):
366 if not self.dolt.status().is_clean:
367 # TODO better error messages
--> 368 raise ValueError("DoltSQLServerManager does not support ")
369 if self.server_config.branch:
370 current_branch, _ = self.dolt.branch()
```

After committing changes:

```
$ dolt status
On branch test_data_types
nothing to commit, working tree clean
```

it's working fine.