Can't compile without AST

* `py-solc` Version: 3.1.0

### What was wrong?

Calling `compile_source()` or `compile_files()` without including `ast` in the `output_values` parameter causes a `KeyError` here: https://github.com/ethereum/py-solc/blob/d0a9efad5a38c7b4143fb9c83b7dd0f5d1a88656/solc/main.py#L69

The reason is that `_parse_compiler_output()` unconditionally expects `sources` to be in the `solc` output json which isn't the case when `ast` isn't included.

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