Allow MD5 checksums on FIPS-enabled systems

MD5 is used for upload and download checksums, to compare object content against its ETag, and to skip uploads of unchanged files. On a FIPS-enabled system hashlib refuses MD5 unless the caller marks it as not used for security, so these operations fail with “unsupported hash algorithm” errors.

Add an md5() helper to swiftclient.utils that passes usedforsecurity through (dropping it on python < 3.9, which lacks the keyword) and use it with usedforsecurity=False everywhere swiftclient hashes content. Swift itself already does this, through swift.common.utils.md5().

The unit tests compute expected checksums with MD5 as well, so give them an md5() helper that sets usedforsecurity=False; otherwise they fail on a FIPS-enabled node.