bpo-39395: putenv() and unsetenv() always available (GH-18135)

The os.putenv() and os.unsetenv() functions are now always available.

On non-Windows platforms, Python now requires setenv() and unsetenv()
functions to build.

Remove putenv_dict from posixmodule.c: it's not longer needed.
diff --git a/configure b/configure
index e966836..85120e4 100755
--- a/configure
+++ b/configure
@@ -11548,9 +11548,9 @@
  memrchr mbrtowc mkdirat mkfifo \
  madvise mkfifoat mknod mknodat mktime mremap nice openat pathconf pause pipe2 plock poll \
  posix_fallocate posix_fadvise posix_spawn posix_spawnp pread preadv preadv2 \
- pthread_condattr_setclock pthread_init pthread_kill putenv pwrite pwritev pwritev2 \
+ pthread_condattr_setclock pthread_init pthread_kill pwrite pwritev pwritev2 \
  readlink readlinkat readv realpath renameat \
- sem_open sem_timedwait sem_getvalue sem_unlink sendfile setegid setenv seteuid \
+ sem_open sem_timedwait sem_getvalue sem_unlink sendfile setegid seteuid \
  setgid sethostname \
  setlocale setregid setreuid setresuid setresgid setsid setpgid setpgrp setpriority setuid setvbuf \
  sched_get_priority_max sched_setaffinity sched_setscheduler sched_setparam \
@@ -11558,7 +11558,7 @@
  sigaction sigaltstack sigfillset siginterrupt sigpending sigrelse \
  sigtimedwait sigwait sigwaitinfo snprintf strftime strlcpy strsignal symlinkat sync \
  sysconf tcgetpgrp tcsetpgrp tempnam timegm times tmpfile tmpnam tmpnam_r \
- truncate uname unlinkat unsetenv utimensat utimes waitid waitpid wait3 wait4 \
+ truncate uname unlinkat utimensat utimes waitid waitpid wait3 wait4 \
  wcscoll wcsftime wcsxfrm wmemcmp writev _getpty rtpSpawn
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`