adds Babel to build requirements

[cfm]

For freedomofpress/securedrop-client#1294 towards freedomofpress/securedrop-client#1283:

1. adds Babel 2.9.1 to build requirements; and
2. brazenly signs the resulting checksums, even though I am not a maintainer, so that freedomofpress/securedrop-client@7959f37 can use this branch in testing. :-)

IMPORTANT. At a minimum, a maintainer will need to rebase out 1c12f46 and resign the checksums as usual:

$ # now let us sign the list of sha256sums
$ gpg --armor --output bootstrap-sha256sums.txt.asc --detach-sig  bootstrap-sha256sums.txt
$ # We can even verify if we want
$ BOOTSTRAP=true ./scripts/verify-sha256sum-signature
$ # Update the build-requirements.txt file
$ PKG_DIR=$PWD BOOTSTRAP=true ./scripts/update-requirements

Test plan

In addition to the usual requirements for a maintainer to approve a new or updated dependency here:

1. CI passes here
2. CI passes for freedomofpress/securedrop-client@7959f37

[sssoleileraaa]

Review in progress... just double-checking with @kushaldas first about intentions around diff reviewing the more recent addition of the bootstrap deps

[kushaldas]

@creviera We should do diff-review for any changes to bootstrap. Also the wheels should be rebuild to verify and hash(s) should be matched against upstream.

[sssoleileraaa]

This is currently blocked by #274

[kushaldas]

The update is good.

• CI is passing
• There are diff-reviews

Approved.