update CHANGELOG and bump version to 4.2.1

Author: beniwohli

CHANGELOG.md

@@ -1,7 +1,7 @@
 # Changelog
 
-## Unreleased
-[Check the diff](https://github.com/elastic/apm-agent-python/compare/v4.2.0...master)
+## v4.2.1 
+[Check the diff](https://github.com/elastic/apm-agent-python/compare/v4.2.0...v4.2.1)
  * fixed an issue with the certificate pinning feature introduced in 4.2.0 (#433, #434)
  * fixed incompatibility with eventlet introduced in 4.2.0 (#435, #436)
 

elasticapm/version.py

@@ -28,5 +28,5 @@
 #  OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 #  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-__version__ = (4, 2, 0)
+__version__ = (4, 2, 1)
 VERSION = ".".join(map(str, __version__))