Updates the script to add multiple wheel hashes

[kushaldas]

Now we can have multiple wheels hashes into the build-requirement
script, for python3.5 and python3.7

How to test?

On a Buster VM/system

• PKG_DIR=/home/user/code/securedrop-client make requirements
• PKG_DIR=/home/user/code/securedrop-client make requirements

This should create a build-requirements.txt with multiple hashes for sqlalchemy and MarkupSafe wheels.

[redshiftzero]

this should be reviewed after #86, correct? (since the sha256sums.txt on this branch only has one wheel per dependency)

[redshiftzero]

looks good! note the two checkboxes in the test plan are the same? just want to check I'm not missing something

after #87 is merged we can add tests when we make changes like this

[redshiftzero]

Due to the make requirements checks that we have in CI (that ensure that the build requirements are up to date given what is in requirements.txt), when we changed make requirements to produce different build requirements - i.e. since build-requirements.txt should have wheels for both 3.5 and 3.7 - we also need to update the build-requirements.txt in repos with this check or CI build jobs will fail