mypy: Updates type hints for morepath 1.0

[Daverball]

Preparation for the eventual morepath 1.0 release.

Switch back to a regular version pin from directly installing the git repository before merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.51%. Comparing base (d52fcbb) to head (73f5388).
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/agency/app.py	97.79% <100.00%> (+0.01%)	⬆️
src/onegov/agency/request.py	100.00% <100.00%> (ø)
src/onegov/agency/security.py	99.24% <ø> (ø)
src/onegov/core/__init__.py	100.00% <100.00%> (ø)
src/onegov/core/directives.py	99.28% <100.00%> (ø)
src/onegov/core/framework.py	93.13% <100.00%> (-0.02%)	⬇️
src/onegov/core/identity.py	100.00% <100.00%> (ø)
src/onegov/core/request.py	97.17% <100.00%> (ø)
src/onegov/core/security/identity_policy.py	100.00% <100.00%> (ø)
src/onegov/election_day/app.py	99.00% <100.00%> (+<0.01%)	⬆️
... and 29 more

... and 2 files with indirect coverage changes

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