Update get_total() to support Coverage 7.5 (#28)

In Coverage 7.5, the `display_covered` method of Numbers was turned into a standalone function, so the `get_total()` method stopped working.

Fixes #29.

coverage_badge/__main__.py

@@ -67,13 +67,16 @@ def pc_covered(self):
                 return self.percent
 
         return Precision(total).pc_covered_str
-    else:  # Coverage 6.x
+    elif hasattr(coverage.results.Numbers, 'display_covered'): # Coverage 6.x < 7.5
         # NOTE: Precision is no longer set globally in the
         # `coverage.results.Numbers` class. Instead the precision must be
         # passed in as the first argument. We pull the precision from the
         # `coverage.Coverage` object because it should pull the correct
         # precision from the local .coveragerc file.
         return coverage.results.Numbers(precision=cov.config.precision).display_covered(total)
+    else: # Coverage >= 7.5
+        return coverage.results.display_covered(total, cov.config.precision)
+
 
 
 def get_color(total):