update spdx-tools dependency to 0.8.1

Main feature of that update is much faster validation
of large SBOMs, so that validation via spdx-tools
becomes viable.

Signed-off-by: Armin Tänzer <armin.taenzer@tngtech.com>

requirements.txt

@@ -18,5 +18,5 @@ GitPython~=3.1
 prettytable~=3.8
 packageurl-python>=0.11.1
 license-expression>=30.1
-spdx-tools>=0.8.0
+spdx-tools>=0.8.1