Consider more strict `mypy`

[johny-b]

Before https://github.com/golemfactory/golem-core-python/pull/83/files#diff-50c86b7ed8ac2cf95bd48334961bf0530cdc77b5a56f852c5c61b89d735fd711 we had almost-strict mypy. Now it's much weaker. BUT: returning to the strict version shouldn't be very hard - there are not that many errors and most are in parts copied from yapapi:

~/golem-core-python {main} $ mypy . --strict --allow-any-generics --implicit-reexport --allow-untyped-calls 2>&1 | cut -d ':' -f1 | sort | uniq
Found 48 errors in 11 files (checked 114 source files)
examples/task_api_draft/examples/yacat.py
examples/task_api_draft/examples/yacat_no_business_logic.py
golem_core/core/activity_api/resources/pooling_batch.py
golem_core/core/market_api/resources/demand/demand_builder.py
golem_core/core/market_api/resources/demand/demand_offer_base/model.py
golem_core/core/resources/base.py
golem_core/managers/payment/default.py
golem_core/utils/storage/gftp.py
tests/unit/test_demand_builder.py
tests/unit/test_demand_builder_model.py
tests/unit/utils/test_storage.py