FTBFS: no default /usr/bin/python

uwsgi-plugin-php is silently failing to build the php plugin:

$ uwsgi --build-plugin /usr/src/uwsgi/plugins/php
$ echo $?
1

strace shows it searching for "python" in many different places, and giving up:
29616 execve("/usr/sbin/python", ["python", ".uwsgi_plugins_builder/uwsgiconf"..., "--extra-plugin", "/usr/src/uwsgi/plugins/php"], 0x555a9d65f140 /* 44 vars */) = -1 ENOENT (No such file or directory)
29616 execve("/usr/bin/python", ["python", ".uwsgi_plugins_builder/uwsgiconf"..., "--extra-plugin", "/usr/src/uwsgi/plugins/php"], 0x555a9d65f140 /* 44 vars */) = -1 ENOENT (No such file or directory)
29616 execve("/sbin/python", ["python", ".uwsgi_plugins_builder/uwsgiconf"..., "--extra-plugin", "/usr/src/uwsgi/plugins/php"], 0x555a9d65f140 /* 44 vars */) = -1 ENOENT (No such file or directory)
...

Looking at the uwsgi src, we see:
core/plugins_builder.c:uwsgi_build_plugin()
...
        // now run the python script
        char *argv[6];

        argv[0] = getenv("PYTHON");
        if (!argv[0]) argv[0] = "python";

Therefore, I suggest this fix for uswgi-plugin-php:
diff --git a/debian/rules b/debian/rules
index a426234..885381e 100755
--- a/debian/rules
+++ b/debian/rules
@@ -26,7 +26,7 @@ OUR_BINARY_VERSION = $(subst -,+,$(UWSGI_VERSION))+$(DEB_VERSION)
        dh $@ --with uwsgi

 override_dh_auto_build:
- uwsgi --build-plugin /usr/src/uwsgi/plugins/php
+ PYTHON=python3 uwsgi --build-plugin /usr/src/uwsgi/plugins/php
        help2man \
                --name 'fast (pure C), self-healing, developer-friendly WSGI server' \
                --section 1 \