multidict 5.1.0 conflicts with sanic==19.12.4

[autumnjolitz]

Describe the bug
multidict is locked to 5.0.0 on Sanic==19.12.x. Various packages like aiohttp use multidict>=5.0.0,<6.0.0a0 as a minor release (or even bugfix) should not break the effective API.

Commit (d0f0e73#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7) for the 20.x series uses this version specifier as well.

Code snippet

Fateweaver:~$ python -m venv test_env
Fateweaver:~$ cd test_env/
Fateweaver:~/test_env$ PIP_EXTRA_INDEX_URL='' ./bin/python -m pip install -U pip
Requirement already satisfied: pip in /Users/autumn/.virtualenvs/cpython36/lib/python3.7/site-packages (20.3.1)
Fateweaver:~/test_env$ PIP_EXTRA_INDEX_URL='' ./bin/python -m pip install multidict==5.1.0 sanic==19.12.4
INFO: pip is looking at multiple versions of multidict to determine which version is compatible with other requirements. This could take a while.
ERROR: Cannot install multidict==5.1.0 and sanic==19.12.4 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested multidict==5.1.0
    sanic 19.12.4 depends on multidict==5.0.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
Fateweaver:~/test_env$ PIP_EXTRA_INDEX_URL='' ./bin/python -m pip install multidict==5.1.0 sanic==20.9.1
INFO: pip is looking at multiple versions of multidict to determine which version is compatible with other requirements. This could take a while.
ERROR: Cannot install multidict==5.1.0 and sanic==20.9.1 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested multidict==5.1.0
    sanic 20.9.1 depends on multidict==5.0.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
Fateweaver:~/test_env$

Expected behavior
Sanic plays well with multdict==5.1.0

Environment (please complete the following information):

• OS: OSX, 18.7.0
• Version 19.12.4, 20.9.1

Additional context

[ashleysommer]

Hi @autumnjolitz
This is a known issue and is already fixed in master: #1969
It will be released as part of the Sanic 20.12 release.

[autumnjolitz]

@ashleysommer Will this be a part of the 19.x series?

[ahopkins]

@autumnjolitz Yes. I will release this with 19.X at the same time we release 20.X. Probably next week.

[autumnjolitz]

Hey @ahopkins , @ashleysommer ,

I've been running with an "multidict>=5.0.0,<6.0.0a0" in my fork https://github.com/autumnjolitz/sanic/tree/v19.12.5 for several months without issue. I've noticed that this change never made it back to the 19.12LTS branch.

Would it be possible for autumnjolitz@f7b46d9 to make it into 19.12 LTS?

[ahopkins]

See #2027 which I am preparing for release now.