Pyhmmer

Added
- `query` property `TopHits` referencing the original object used to create the `TopHits` [76](https://github.com/althonos/pyhmmer/issues/76).

Changed
- Require the query object to create a `TopHits` object.
- Make `TopHits` generic over its `query` property.
- Deprecate old query properties of `TopHits` (`query_name`, `query_length`, `query_accession`).

Removed
- Detection of SSE flush from `setup.py` ([71](https://github.com/althonos/pyhmmer/issues/71)).

Added
- Detection of SSE flush modes to `setup.py` for possible performance gains on x86 platforms.

Changed
- Migrate documentation to `pydata-sphinx-theme`.

Fixed
- Documentation examples not using permanent resource links.

Changed
- Allow `AlphabetMismatch` error to allow for an unknown *actual* alphabet.
- Make `HMMFile` and `HMMPressedFile` raise `AlphabetMismatch` on files with mixed alphabets.

Fixed
- Avoid calling `fclose` with null pointers in `Sequence.write` and `MSA.write`.

Fixed
- `HMM.__setstate__` not properly extracting the cutoff from `pickle` state for some HMMs ([67](https://github.com/althonos/pyhmmer/issues/67)).

Changed
- Update and remove some test files to reduce size of distributed package data.

Fixed
- Compilation of Easel and HMMER code not using SSE4.1 extensions.

Fixed
- Implement `write` function for `fopencookie` with `off_t` instead of `off64_t` for compatibility.
- Fix handling of NULL buffers passed to `read` and `write` methods of `fopencookie`.