Add abi3 feature to pyo3.

Seems like the latest pyo3 version (0.20.3 at the date of writing) introduces a maximum version check which can make the our CI to fail.

This can be turned off by setting the `abi3` feature, so that's what I did.

More info: PyO3/pyo3#3821

btw, why are you reading this commit message? I almost never write commit messages this long, so it an interesting coincidence that you ended up here.

Please reach me and tell me how you ended up diggging this, so we can have a laugh together

Cargo.toml

@@ -23,7 +23,7 @@ crate-type = ["cdylib", "staticlib", "rlib"]
 
 [dependencies]
 regex = "1.10.2"
-pyo3 = { version = "0.20.0", optional = true }
+pyo3 = { version = "0.20.0", optional = true, features = ["abi3", "abi3-py37"]}
 lazy_static = "1.4.0"
 
 [features]