Python 3.6 EOL

[martinburchell]

Python 3.6 reaches end-of-life on 23rd December 2021.
https://www.python.org/downloads/

We should standardise on a later version (Travis CI refers to 3.6, anything else?) and make sure all our server instances are updated.

[RudolfCardinal]

Python 3.9 works fine with the self-tests (as does 3.6 still), after some fixes.

1. Bumping pendulum from 2.1.0 to 2.1.2 to avoid this installation bug: Could not build wheels for pendulum which use PEP 517 python-pendulum/pendulum#454

• ... which required a test change in SqlaColtypesTest._assert_duration_equal from assert a == b to assert a.total_seconds() == b.total_seconds(), relating to Fix the computation of total seconds with years and months python-pendulum/pendulum#482 -- without this change there were discrepancies on the days/weeks fields, even if the total duration was identical. I'm not convinced that Pendulum has done the right thing entirely here, but I've gone with it, and updated cardinal_pythonlib accordingly.

The failure was:

_______________________________________ SqlaColtypesTest.test_iso_duration_field _______________________________________
cc_modules/tests/cc_sqla_coltypes_tests.py:197: in test_iso_duration_field
    self._assert_duration_equal(rows[0][duration_col], d1)
cc_modules/tests/cc_sqla_coltypes_tests.py:147: in _assert_duration_equal
    assert a.total_seconds() == b.total_seconds(), f"{a!r} != {b!r}"
E   AssertionError: Duration(years=1, months=3, weeks=65, days=0, seconds=3, microseconds=4) != Duration(years=1, months=3, seconds=3, microseconds=4)
E   assert 78624003.000004 == 39312003.000004
E     +78624003.000004
E     -39312003.000004
-------------------------------------------------- Captured log call ---------------------------------------------------
INFO     sqlalchemy.engine.base.Engine:log.py:110 SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
INFO     sqlalchemy.engine.base.Engine:log.py:110 ()
INFO     sqlalchemy.engine.base.Engine:log.py:110 SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
INFO     sqlalchemy.engine.base.Engine:log.py:110 ()
INFO     sqlalchemy.engine.base.Engine:log.py:110 
CREATE TABLE testtable (
    id INTEGER NOT NULL, 
    duration_iso VARCHAR(29), 
    PRIMARY KEY (id)
)


INFO     sqlalchemy.engine.base.Engine:log.py:110 ()
INFO     sqlalchemy.engine.base.Engine:log.py:110 COMMIT
INFO     sqlalchemy.engine.base.Engine:log.py:110 INSERT INTO testtable (id, duration_iso) VALUES (?, ?), (?, ?), (?, ?)
INFO     sqlalchemy.engine.base.Engine:log.py:110 (1, 'P1Y3MT39312003.000004S', 2, 'P0Y0MT987.654321S', 3, '-P0Y0MT432000.0S')
INFO     sqlalchemy.engine.base.Engine:log.py:110 COMMIT
INFO     sqlalchemy.engine.base.Engine:log.py:110 SELECT testtable.id, testtable.duration_iso 
FROM testtable ORDER BY testtable.id
INFO     sqlalchemy.engine.base.Engine:log.py:110 ()

The problem is that in pendulum 2.1.0, you get this:

d1 = Duration(years=1, months=3, seconds=3, microseconds=4)
d1  # Duration(years=1, months=3, days=0, seconds=3, microseconds=4)
d1.total_seconds()  # 3.000004

and in pendulum 2.1.2, you get this:

d1 = Duration(years=1, months=3, seconds=3, microseconds=4)
d1  # Duration(years=1, months=3, seconds=3, microseconds=4)
d1.total_seconds()  # 39312003.000004

Because cardinal_pythonlib worked with the older version of pendulum, with the newer version you got an ISO-8601 duration like:

P1Y3MT39312003.000004S

whereas it should be

P1Y3MT3.000004S

2. Fixing an internal import from the enum standard library, by copying its code, in cardinal_pythonlib==1.1.5

Ubuntu prerequites are sudo apt-get install python3.9 python3.9-venv python3.9-dev.

I guess we don't have to increase MINIMUM_PYTHON_VERSION yet. However, should we move to Docker generally for our servers so we have a standard image?

I've not changed .travis.yml yet.

There are deprecation warnings made apparent by ``export

Changes are in #175

[RudolfCardinal]

Minimum Python version bumped to 3.7 as of CamCOPS v2.4.12 (per cc_pythonversion.py).