Check for SpecifierSet syntactic consistency

Hi,

is it possible to check whether a `SpecifierSet` is "satisfiable" by any version?

Example:

The specifier set `"<1.0, >=1.0"` is clearly not satisfiable by any version number.
```python
from packaging.specifiers import SpecifierSet
specifier_set = SpecifierSet("<1.0, >=1.0")
```

I'd like an operation like:
```
specifier_set.satisfiable()  # returns False
```
