build-integration-branch: convert to argparse

build-integration-branch: convert to argparse

Convert build-integration-branch to use the stdlib argparse module.

Argparse is:
* Part of the python standard library and available since 3.2
* Well documented as a stdlib component
* Widely used
* Fairly simple and direct

docopt is:
* Clever
* Not documented as a dependency of this script (so I bet most users
  are relying on the fallback behavior)
* Of questionable maintenance status with:
  - No releases since 2014
  - Only four PRs merged since 2019
  - Last merged PR was merged, recommending an alternate repo, and then
    disappeared from the commit history of the master branch, indicating
    a possible maintainership/status discrepancy
  - Only a couple of commits merged since 2018 (visible on github)
* In my opinion: not particularly ergonomic esp. wrt dictionary based
  key access

I feel pretty comfortable making this conversion as I think it will
make the script easier to maintain and extend.

Signed-off-by: John Mulligan <jmulligan@redhat.com>