Prepare v3.2.0 release

[khaeru]

• Adjust to mypy 0.800, including adjustments needed by message-ix.
• Increase JPype minimum version to 1.2.1 per discussion at https://github.com/iiasa/message_ix/issues/386#issuecomment-760791620

How to review

Note that CI checks all pass

PR checklist

• ~Add or expand tests.~ N/A, packaging changes only
• [x] Add, expand, or update documentation.
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #394 (8e88ee5) into master (277f2b6) will increase coverage by 0.0%. The diff coverage is 75.0%.

@@          Coverage Diff           @@
##           master    #394   +/-   ##
======================================
  Coverage    96.8%   96.8%           
======================================
  Files          46      46           
  Lines        5335    5337    +2     
======================================
+ Hits         5169    5171    +2     
  Misses        166     166           

Impacted Files	Coverage Δ
ixmp/tests/core/test_scenario.py	100.0% <ø> (ø)
ixmp/testing.py	78.3% <50.0%> (ø)
ixmp/cli.py	98.7% <100.0%> (+<0.1%)	:arrow_up:
ixmp/tests/backend/test_jdbc.py	84.0% <100.0%> (+0.1%)	:arrow_up:

[khaeru]

The codecov/patch check failure is acceptable, as it pertains to test code only.