bpo-45621: Small changes to mmap

[tjguk]

Following issue40915 a few small items still need to be addressed, mostly cosmetic / naming:

• A comment should have been removed but wasn't
• A comment should have been added but wasn't
• The use of the string "TEST" should be avoided in tests particularly where there is a shared namespace and therefore the chance of a collision

https://bugs.python.org/issue45621

[eryksun]

How about including context in the name? For example:

tagname_prefix = f'python_{os.getpid()}_test_mmap'
def random_tagname(suffix_length=10):
    suffix = ''.join(random.choices(string.ascii_uppercase, k=suffix_length))
    return f'{tagname_prefix}_{suffix}'

[tjguk]

Ok, why not? (And thanks for the suggestion of random.choices: TIL)