Enable Flake8 on CI and fix issues

[Flamefire]

See easybuilders/easybuild#643

[boegel]

lgtm

[boegel]

@boegelbot please test @ generoso

[boegelbot]

@boegel: Request for testing this PR well received on generoso

PR test command 'EB_PR=11147 EB_ARGS= /apps/slurm/default/bin/sbatch --job-name test_PR_11147 ~/boegelbot/eb_from_pr_upload_generoso.sh' executed!

• exit code: 0
• output:

Submitted batch job 5091

Test results coming soon (I hope)...

- notification for comment with ID 677633848 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 6 out of 6 (6 easyconfigs in this PR)
node2711.swalot.os - Linux centos linux 7.8.2003, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 2.7.5
See https://gist.github.com/9f28164eff219e1aba413a18d4f72b83 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 6 out of 6 (6 easyconfigs in this PR)
node3407.kirlia.os - Linux centos linux 7.8.2003, x86_64, Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz (cascadelake), Python 2.7.5
See https://gist.github.com/ebef5d458b99b96d8907085508cdcdca for a full test report.

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 6 out of 6 (6 easyconfigs in this PR)
generoso - Linux centos linux 8.2.2004, x86_64, Intel(R) Xeon(R) CPU E5-2667 v3 @ 3.20GHz (haswell), Python 3.6.8
See https://gist.github.com/f1eaef696396f84c3ce36302723da241 for a full test report.

[boegel]

lgtm

[boegel]

Going in, thanks @Flamefire!