Prep datastore-1.1.0 release.

[tseaver]

Draft release notes

google-cloud-datastore-1.1.0

• Update google-cloud-core dependency to ~= 0.25.
• Add Key.(to|from)_legacy_urlsafe. (PR Adding datastore Key.(to|from)_legacy_urlsafe. #3491)

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Not included in notes:

• Re-enable pylint in info-only mode for all packages (Re-enable pylint in info-only mode for all packages #3519)
• Vision semi-GAPIC (Vision semi-GAPIC #3373) (docstring tweaks)
• Add better documentation for support types in datastore Entity. (Add better documentation for support types in datastore Entity. #3363)
• Ignore tests (rather than unit_tests) in setup.py files. (Ignore tests (rather than unit_tests) in setup.py files. #3319)
• Adding check (in datastore) that setup.py README is valid RST. (Adding check (in datastore) that setup.py README is valid RST. #3316)

[tseaver]

Ugh. Requires merge of #3526 to bump core to 0.25.0.

[dhermes]

LGTM

[tseaver]

Rebuilding now that the core change is in.