Add abi3-py* features

Cargo.toml

@@ -35,9 +35,13 @@ rustversion = "1.0"
 [features]
 default = ["macros"]
 macros = ["ctor", "indoc", "inventory", "paste", "pyo3cls", "unindent"]
-# Use the Python limited API. See https://www.python.org/dev/peps/pep-0384/ for
-# more.
+# Use the Python limited API. See https://www.python.org/dev/peps/pep-0384/ for more.
 abi3 = []
+# With abi3, we can manually set the minimum Python version.
+abi3-py36 = ["abi3"]
+abi3-py37 = ["abi3-py36"]
+abi3-py38 = ["abi3-py37"]
+abi3-py39 = ["abi3-py38"]
 
 # Optimizes PyObject to Vec conversion and so on.
 nightly = []

build.rs

@@ -9,7 +9,10 @@ use std::{
     str::FromStr,
 };
 
-const PY3_MIN_MINOR: u8 = 5;
+/// Minimum required Python version.
+const PY3_MIN_MINOR: u8 = 6;
+/// Maximum Python version that can be used as minimum required Python version with abi3.
+const ABI3_MAX_MIN_MINOR: u8 = 9;
 const CFG_KEY: &str = "py_sys_config";
 
 type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;
@@ -770,12 +773,24 @@ fn configure(interpreter_config: &InterpreterConfig) -> Result<String> {
         bail!("Python 2 is not supported");
     }
 
-    if env::var_os("CARGO_FEATURE_ABI3").is_some() {
-        println!("cargo:rustc-cfg=Py_LIMITED_API");
+    fn get_abi3_min_python() -> Option<u8> {
+        for i in PY3_MIN_MINOR..=ABI3_MAX_MIN_MINOR {
+            if env::var_os(format!("CARGO_FEATURE_ABI{}", i)).is_some() {
+                return Some(i);
+            }
+        }
+        None
     }
 
-    if let Some(minor) = interpreter_config.version.minor {
-        for i in 6..=minor {
+    let minor = if env::var_os("CARGO_FEATURE_ABI3").is_some() {
+        println!("cargo:rustc-cfg=Py_LIMITED_API");
+        get_abi3_min_python().or(interpreter_config.version.minor)
+    } else {
+        interpreter_config.version.minor
+    };
+
+    if let Some(minor) = minor {
+        for i in PY3_MIN_MINOR..=minor {
             println!("cargo:rustc-cfg=Py_3_{}", i);
             flags += format!("CFG_Py_3_{},", i).as_ref();
         }