Lmdb

* Migrate CI pipeline from Travis and AppVeyor to Github Actions. Now
includes comprehensive testing across 4 dimensions (OS, Python version,
cpython/CFFI, pure/with mods). Also includes publishing to PyPI.

* Prevent invalid flag combinations when creating a database.

* Add a Cursor.getmulti method with optional buffer support. Contributed by
Will Thompson <willsthompsongmail.com>.

* Upgrade underlying LMDB to 0.9.27.

* Start of new semantic versioning scheme. This would be a minor version
bump from the 0.99 release if it were semantically versioned.

* Allow environment copy to take a passed-in transaction. This is the
first released feature that requires a (very small) patch to the
underlying C library. By default, the patch will be applied unless
this module is built with LMDB_PURE environment variable set.

* Fix lmdb.tool encoding issues.

* Fix -l lmdb invocation issue.

* Minor documentation improvements.

* Update LMDB to version 0.9.24.

* Update for Python 3.9 (current release candidate) support.

* Resolve a bug when using cursor.putmulti and append=True on dupsort DBs.

* Allow _Database.flags method to take no arguments since the one argument
wasn't being used.

* Fix that a duplicate argument to a lmdb method would cause an assert.

* Solaris needs include "python.h" as soon as possible. Fix
contributed by Jesús Cea.

* Fix crash under debug cpython when mdb_cursor_open failed

* Fix a missed GIL unlock sequence. Reported by ajschorr.

* Fix argv check in JEP (cpython under Java) environment. Contributed by
de-code.

* First release under new maintainer, Nic Watson.

* Doc updates.

* More removal of code for now-unsupported Python versions.

* Only preload the value with the GIL unlocked when the value is actually
requested. This significantly improves read performance to retrieve keys
with large values when the value isn't retrieved. Reported by Dan Patton.