Unsatisfied with the solutions presented here so far, I released the python. She was effective. This solution doesn't require setuid permissions or any actually-insane monkey-patching with shared libraries and `LD_LIBRARY_PATH`.

 #!/usr/bin/env python

 from sys import argv, stdin, stdout
 import os

 if len(argv) < 2:
 print(
 """A tragically beautiful piece of hackery, made to fool programs like ls,
 grep, rg, and fd into thinking they're actually connected to a terminal.
 Its usage:

 pty command [arg1 arg2 ...]

 examples:
 pty ls --color | less -r
 ps -eF | pty rg python | less -r""")
 exit(255)

 # I've had problems with python's File objects at this low a level, so
 # we're going to use integers to specify all files in this script.
 stdin = stdin.buffer.fileno()
 stdout = stdout.buffer.fileno()

 # Save the parent stdin and ensure the child inherits it. We're going
 # to need this later.
 parent_stdin = os.dup(stdin)
 os.set_inheritable(parent_stdin, True)
 pid, ptm = os.forkpty()
 if pid == 0:
 # The child runs this.
 # forkpty maps the pty slave to stdin, stdout, and stderr, but
 # we want the child to receive from our stdin, in order for things
 # like `ps -eF | pty rg python` to work. So, overwrite the child's
 # stdin with that of the parent's.
 os.dup2(parent_stdin, stdin)
 os.execvp(argv[1], argv[1:]) # Replace this entire process with argv[1].
 else:
 # The parent runs this.
 while True:
 try:
 chunk = os.read(ptm, 4096)
 except OSError:
 break
 os.write(stdout, chunk)

 wait_pid, status = os.waitpid(pid, 0)
 exit(status >> 8)