PyPy Support

blake2b-py currently doesn't support compiling for pypy 3.7 without additional configuration (assuming rust is installed)

```
$ pip install blake2b-py
...bash
Building wheels for collected packages: blake2b-py
Building wheel for blake2b-py (PEP 517) ... error
ERROR: Command errored out with exit status 1:
command: ~/.pyenv/versions/pypy3.7-7.3.4/envs/ape-pypy/bin/pypy3 ~/.pyenv/versions/pypy3.7-7.3.4/envs/ape-pypy/site-packages/pip/_vendor/pep517/in_process/_in_process.py build_wheel /tmp/tmpkqxloceo
cwd: /tmp/pip-install-36bcisym/blake2b-py_5d24a7fa36444f4f9f3f16e5f806ed71
Complete output (47 lines):
Running `maturin pep517 build-wheel -i ~/.pyenv/versions/pypy3.7-7.3.4/envs/ape-pypy/bin/pypy3`
Compiling proc-macro2 v1.0.26
Compiling unicode-xid v0.2.2
Compiling syn v1.0.72
Compiling proc-macro-hack v0.5.19
Compiling serde_derive v1.0.125
Compiling memchr v2.4.0
Compiling ryu v1.0.5
Compiling serde v1.0.125
Compiling serde_json v1.0.64
Compiling itoa v0.4.7
Compiling autocfg v1.0.1
Compiling regex-syntax v0.6.25
Compiling version_check v0.9.3
Compiling libc v0.2.94
Compiling inventory v0.1.10
Compiling unindent v0.1.7
Compiling lazy_static v1.4.0
Compiling spin v0.5.2
Compiling num-traits v0.2.14
Compiling aho-corasick v0.7.18
Compiling quote v1.0.9
Compiling paste-impl v0.1.18
Compiling paste v0.1.18
Compiling regex v1.5.3
Compiling pyo3-derive-backend v0.8.5
Compiling ghost v0.1.2
Compiling inventory-impl v0.1.10
Compiling indoc-impl v0.3.6
Compiling ctor v0.1.20
Compiling pyo3cls v0.8.5
Compiling indoc v0.3.6
Compiling pyo3 v0.8.5
error: failed to run custom build command for `pyo3 v0.8.5`

Caused by:
process didn't exit successfully: `/tmp/pip-install-36bcisym/blake2b-py_5d24a7fa36444f4f9f3f16e5f806ed71/target/release/build/pyo3-412cf57dbee04baa/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'Error: pyo3 requires a nightly or dev version of Rust.', ~/.cargo/registry/src/github.com-1ecc6299db9ec823/pyo3-0.8.5/build.rs:542:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
💥 maturin failed
Caused by: Failed to build a native library through cargo
Caused by: Cargo build finished with "exit code: 101": `cargo rustc --message-format json --manifest-path Cargo.toml --release --lib --`
🍹 Building a mixed python/rust project
🔗 Found pyo3 bindings
🐍 Found PyPy 3.7 at ~/.pyenv/versions/pypy3.7-7.3.4/envs/ape-pypy/bin/pypy3
Error: command ['maturin', 'pep517', 'build-wheel', '-i', '~/.pyenv/versions/pypy3.7-7.3.4/envs/ape-pypy/bin/pypy3'] returned non-zero exit status 1
----------------------------------------
ERROR: Failed building wheel for blake2b-py
Failed to build blake2b-py
ERROR: Could not build wheels for blake2b-py which use PEP 517 and cannot be installed directly
```

---

Easy solution is to just release binary wheels for pypy (3.7 preferred, but 3.6 too maybe)