Python build failed on MacOs Big Sur 11.2.3

Relates to #93 and #94

After updating to MacOs 11.2.3 I cannot build python.

I've reinstalled all Brew dependencies:

`brew reinstall bzip2 lbzip2 lzlib openssl zlib`

I've set environment variables, and run `asdf install` with variables directly:

```bash
LDFLAGS="-L/usr/local/opt/bzip2/lib -L/usr/local/opt/zlib/lib -L/usr/local/opt/openssl@1.1/lib" CFLAGS="-I/usr/local/opt/bzip2/include -I/usr/local/opt/zlib/include -I/usr/local/opt/openssl@1.1/include -I$(xcrun --show-sdk-path)/usr/include -Wno-implicit-function-declaration" asdf install python 3.8.8
```

This is the current result of running with variables;

```bash
python-build 3.8.8 /Users/anders/.asdf/installs/python/3.8.8
python-build: use openssl@1.1 from homebrew
python-build: use readline from homebrew
Downloading Python-3.8.8.tar.xz...
-> https://www.python.org/ftp/python/3.8.8/Python-3.8.8.tar.xz
Installing Python-3.8.8...
python-build: use readline from homebrew
python-build: use zlib from xcode sdk

BUILD FAILED (OS X 11.2.3 using python-build 1.2.23-75-g80e418ec)

Inspect or clean up the working tree at /var/folders/tl/06ddx4rj5tj_5h3gpvvtxtbm0000gn/T/python-build.20210317113113.24543
Results logged to /var/folders/tl/06ddx4rj5tj_5h3gpvvtxtbm0000gn/T/python-build.20210317113113.24543.log

Last 10 log lines:
File "/private/var/folders/tl/06ddx4rj5tj_5h3gpvvtxtbm0000gn/T/python-build.20210317113113.24543/Python-3.8.8/Lib/distutils/command/build_ext.py", line 340, in run
self.build_extensions()
File "./setup.py", line 320, in build_extensions
self.detect_modules()
File "./setup.py", line 1695, in detect_modules
self.detect_readline_curses()
File "./setup.py", line 930, in detect_readline_curses
(tuple(int(n) for n in dep_target.split('.')[0:2])
AttributeError: 'int' object has no attribute 'split'
make: *** [sharedmods] Error 1
```

指定された環境変数を使用して macOS 11.2.3 上で `asdf install python 3.8.8` を再現し、その後、python-build のログと `Python-3.8.8/Lib/distutils/command/build_ext.py` および `setup.py` にあるトレースバックを調査する。特に `detect_modules` と `detect_readline_curses` を確認する。この環境で Python のビルドが正常に完了すれば完了とする。

索引モデルが issue の本文から書いたものです。