Build _dbm as a shared module (#838)

Partially addresses #836.

Author: geofft

cpython-unix/extension-modules.yml

@@ -246,6 +246,8 @@ _dbm:
     - name: db
       targets:
         - .*-unknown-linux-.*
+  # Allow people who want to avoid the Sleepycat license to remove _dbm entirely.
+  build-mode: shared
 
 _decimal:
   sources:

src/validation.rs

@@ -913,7 +913,7 @@ const GLOBAL_EXTENSIONS_WINDOWS_PRE_3_13: &[&str] = &["_msi"];
 const GLOBAL_EXTENSIONS_WINDOWS_NO_STATIC: &[&str] = &["_testinternalcapi", "_tkinter"];
 
 /// Extension modules that should be built as shared libraries.
-const SHARED_LIBRARY_EXTENSIONS: &[&str] = &["_crypt", "_ctypes_test", "_tkinter"];
+const SHARED_LIBRARY_EXTENSIONS: &[&str] = &["_crypt", "_ctypes_test", "_dbm", "_tkinter"];
 
 const PYTHON_VERIFICATIONS: &str = include_str!("verify_distribution.py");