Add __all__'s to v20 and v21/__init__.py to ensure that

star-imports from those modules get the right symbols.  "base"
was a problem because it was getting imported via
stix2/__init__.py and overwriting stix2.base, making the actual
stix2/base.py module inaccessible.

stix2/v20/__init__.py

@@ -103,3 +103,33 @@ EXT_MAP = {
         'unix-account-ext': UNIXAccountExt,
     },
 }
+
+
+# Ensure star-imports from this module get the right symbols.  "base" is a
+# known problem, since there are multiple modules with that name and one can
+# accidentally overwrite another.
+__all__ = """
+    Bundle,
+
+    TLP_AMBER, TLP_GREEN, TLP_RED, TLP_WHITE, CustomMarking, ExternalReference,
+    GranularMarking, KillChainPhase, MarkingDefinition, StatementMarking,
+    TLPMarking,
+
+    URL, AlternateDataStream, ArchiveExt, Artifact, AutonomousSystem,
+    CustomExtension, CustomObservable, Directory, DomainName, EmailAddress,
+    EmailMessage, EmailMIMEComponent, File, HTTPRequestExt, ICMPExt,
+    IPv4Address, IPv6Address, MACAddress, Mutex, NetworkTraffic, NTFSExt,
+    PDFExt, Process, RasterImageExt, SocketExt, Software, TCPExt,
+    UNIXAccountExt, UserAccount, WindowsPEBinaryExt,
+    WindowsPEOptionalHeaderType, WindowsPESection, WindowsProcessExt,
+    WindowsRegistryKey, WindowsRegistryValueType, WindowsServiceExt,
+    X509Certificate, X509V3ExtenstionsType,
+
+    AttackPattern, Campaign, CourseOfAction, CustomObject, Identity, Indicator,
+    IntrusionSet, Malware, ObservedData, Report, ThreatActor, Tool,
+    Vulnerability,
+
+    Relationship, Sighting,
+
+    OBJ_MAP, OBJ_MAP_OBSERVABLE, EXT_MAP
+""".replace(",", " ").split()

stix2/v21/__init__.py

@@ -111,3 +111,34 @@ EXT_MAP = {
         'unix-account-ext': UNIXAccountExt,
     },
 }
+
+
+# Ensure star-imports from this module get the right symbols.  "base" is a
+# known problem, since there are multiple modules with that name and one can
+# accidentally overwrite another.
+__all__ = """
+    Bundle,
+
+    TLP_AMBER, TLP_GREEN, TLP_RED, TLP_WHITE, CustomMarking, ExternalReference,
+    GranularMarking, KillChainPhase, LanguageContent, MarkingDefinition,
+    StatementMarking, TLPMarking,
+
+    URL, AlternateDataStream, ArchiveExt, Artifact, AutonomousSystem,
+    CustomExtension, CustomObservable, Directory, DomainName, EmailAddress,
+    EmailMessage, EmailMIMEComponent, File, HTTPRequestExt, ICMPExt,
+    IPv4Address, IPv6Address, MACAddress, Mutex, NetworkTraffic, NTFSExt,
+    PDFExt, Process, RasterImageExt, SocketExt, Software, TCPExt,
+    UNIXAccountExt, UserAccount, WindowsPEBinaryExt,
+    WindowsPEOptionalHeaderType, WindowsPESection, WindowsProcessExt,
+    WindowsRegistryKey, WindowsRegistryValueType, WindowsServiceExt,
+    X509Certificate, X509V3ExtenstionsType,
+
+    AttackPattern, Campaign, CourseOfAction, CustomObject, Grouping, Identity,
+    Indicator, Infrastructure, IntrusionSet, Location, Malware,
+    MalwareAnalysis, Note, ObservedData, Opinion, Report, ThreatActor, Tool,
+    Vulnerability,
+
+    Relationship, Sighting,
+
+    OBJ_MAP, OBJ_MAP_OBSERVABLE, EXT_MAP
+""".replace(",", " ").split()