Upgrade pyyaml to 6.0.1

[juagargi]

Previous versions are incompatible with Cython 3.0.0, which was released just the previous week.
Upgrading to >5.4.1 seems too complex, given that 6.0.1 looks compatible with our code.

See:
yaml/pyyaml#724
yaml/pyyaml#726

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This change is 

[FR4NK-W]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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.dockerignore line 3 at r1 (raw file):

.circleci

We don't need the files in .circleci to be included in the production docker images. See comment on line 222 in file .circleci/config.yml

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README.md line 50 at r1 (raw file):

#         apt install libssl1.0
# - The `psycopg2-binary` package may fail if libpq-dev is not installed.
#         apt install libpq-dev

Isn't also libffi-dev required if building wheel cffi from source?

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.circleci/config.yml line 63 at r1 (raw file):

            python3 -m venv /tmp/venv 2>&1
            . /tmp/venv/bin/activate
            pip3 install -U pip  # need pip >= 19.0 to install cryptography

This should not be required, in the venv pip and pip3 are identical.

Suggestion:

pip

[FR4NK-W]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

[FR4NK-W]

Reviewable status: complete! 1 of 1 LGTMs obtained