Add usedforsecurity=False to usage of md5 for FIPS mode

[agemuend]

When trying to install easybuild on FIPS mode system, I get the following message:

File "/[...]/[...]/eb_tmp/lib/python3.6/site-packages/easybuild/tools/filetools.py", line 129, in
CHECKSUM_TYPE_MD5: lambda p: calc_block_checksum(p, hashlib.md5()),
ValueError: [digital envelope routines: EVP_DigestInit_ex] disabled for FIPS

Apparently hashlib offers a named parameter called usedforsecurity for exactly this case, allowing the use of md5 even on security enabled systems: https://docs.python.org/3/library/hashlib.html

[Micket]

Slight problem; usedforsecurity is new in python 3.9 and even with EB 5.0 we still allow back to python 3.6 compatibility.

I'm surprised this is triggered in python when that code is just defining a lambda function in a dictionary. It hasn't actually ever called hashlib.md5 yet.
We could restructure this code a bit to avoid that, like moving the md5 stuff into it's own function maybe is enough? Then the error would only occur if we actually tried to compute an md5sum, which I don't think we ever do in the main easyconfig repos.