loader: Use MarkupSafe:2.0.1 specifically

The ru.vyarus.use-python will download MarkupSafe 2.1.1 on my laptopn
with Python 3.8. But MarkupSafe removed soft_unicode from 2.1.0, used by
jinja2:2.10.3, and it will cause importing error when running `./gradlew
build` under src/loader directory. Instead of updating jinja2, this CL
try to fix MarkupSafe version for loader to fix building problem.

The release note of MarkupSafe is here:
https://markupsafe.palletsprojects.com/en/2.1.x/changes/#version-2-1-0.

Signed-off-by: utzcoz <utzcoz@outlook.com>

Author: utzcoz

src/loader/build.gradle

@@ -40,6 +40,7 @@ def registry = "${project.repoRoot}/specification/registry/xr.xml"
 // Python is used to generate header files
 python {
     pip "jinja2:2.10.3"
+    pip "MarkupSafe:2.0.1"
     minPythonVersion = "3.4"
 
     environment = ["PYTHONPATH": scriptDir]