Begin incorporating Python unit tests into build

We now build a separate APE binary for each test so they can run in
parallel. We've got 148 tests running fast and stable so far.

third_party/python/Python/fdisinteractive.c

@@ -0,0 +1,28 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=4 sts=4 sw=4 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Python 3                                                                     │
+│ https://docs.python.org/3/license.html                                       │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "third_party/python/Include/pydebug.h"
+#include "third_party/python/Include/pylifecycle.h"
+/* clang-format off */
+
+/*
+ * The file descriptor fd is considered ``interactive'' if either
+ *   a) isatty(fd) is TRUE, or
+ *   b) the -i flag was given, and the filename associated with
+ *      the descriptor is NULL or "<stdin>" or "???".
+ */
+int
+Py_FdIsInteractive(FILE *fp, const char *filename)
+{
+    if (isatty((int)fileno(fp)))
+        return 1;
+    if (!Py_InteractiveFlag)
+        return 0;
+    return (filename == NULL) ||
+           (strcmp(filename, "<stdin>") == 0) ||
+           (strcmp(filename, "???") == 0);
+}