Don't hide the patch version number when it's 0 (#6687)

IMO, two-component version numbers should only be used when a project
doesn't make patch releases at all. Having two version components for
some releases and three for others is just pointless inconsistency.

Two-component version numbers are also inconsistent with our tag names.

cvat/schema.yml

 openapi: 3.0.3
 info:
   title: CVAT REST API
-  version: '2.7'
+  version: 2.7.0
   description: REST API for Computer Vision Annotation Tool (CVAT)
   termsOfService: https://www.google.com/policies/terms/
   contact:

cvat/utils/version.py

@@ -14,7 +14,7 @@ import subprocess
 def get_version(version):
     """Return a PEP 440-compliant version number from VERSION."""
     # Now build the two parts of the version number:
-    # main = X.Y[.Z]
+    # main = X.Y.Z
     # sub = .devN - for pre-alpha releases
     #     | {a|b|rc}N - for alpha, beta, and rc releases
 
@@ -33,9 +33,8 @@ def get_version(version):
     return main + sub
 
 def get_main_version(version):
-    """Return main version (X.Y[.Z]) from VERSION."""
-    parts = 2 if version[2] == 0 else 3
-    return '.'.join(str(x) for x in version[:parts])
+    """Return main version (X.Y.Z) from VERSION."""
+    return '.'.join(str(x) for x in version[:3])
 
 def get_git_changeset():
     """Return a numeric identifier of the latest git changeset.