> Adding `import gc; gc.collect()` doesn't change the outcome afaict

Of course it doesn't. The memory has already been released.
"ru_maxrss" is the maximum memory consumption during the whole process lifetime. Add the following at the end of your script (Linux):

import os, re, resource
print(resource.getrusage(resource.RUSAGE_SELF).ru_maxrss)

with open("/proc/%d/status" % os.getpid(), "r") as f:
    for line in f:
        if line.split(':')[0] in ('VmHWM', 'VmRSS'):
            print(line.strip())


And you'll see that VmRSS has already fallen back to the same level as when the pyc is not recompiled (it's a little bit more, perhaps due to fragmentation):

$ rm -r  __pycache__/; ./python -c "import repro"
19244
VmHWM:	   19244 kB
VmRSS:	   12444 kB
$ ./python -c "import repro"
12152
VmHWM:	   12152 kB
VmRSS:	   12152 kB

("VmHWM" - the HighWater Mark - is the same as ru_maxrss)