Merge pull request MongoEngine#2803 from bagerard/fix_pymongo37

Fix pymongo37 incompatibility

Merge pull request MongoEngine#2800 from bagerard/bug_no_deref_thread…

…_local

Fix bug in mutable thread local related with no_dereference ctx manager

Prepare for 0.28.0

Merge pull request MongoEngine#2740 from bagerard/prepare_0_27

Prepare 0.27

Merge pull request MongoEngine#2727 from bagerard/prepare_0_26_0_release

prepare_0_26_0_release

Merge pull request MongoEngine#2718 from bagerard/prepare_release_0_25_0

improvements for 0 25 0

Merge pull request MongoEngine#2676 from bagerard/prepare_0_24_2

prepare_0_24_2

Merge pull request MongoEngine#2637 from bagerard/prepare_0_24_1

Prepare release 0 24 1

Merge pull request MongoEngine#2623 from bagerard/prepare_release_0_24_0

Prepare release 0 24 0

Merge pull request MongoEngine#2513 from bagerard/prepare_release

Prepare release