[llvm][docs] Update the Python version requirement to 3.8.0 for lit testing on Windows with substitute (virtual) drives.

Following the changes made for:
- https://reviews.llvm.org/D154130: [lit][clang] Avoid realpath on Windows due to MAX_PATH limitations
in commit:
- 05d613e
Python 3.8.0 or newer is now required by at least the following tests when
they are run on Windows from a substitute (virtual) drive. A substitute drive
is often used as a workaround for MAX_PATH limitations on Windows. These tests
are impacted because they use the lit '%{?:real}' path expansion syntax to
expand symbolic links and substitute drives. This path expansion is implemented
with Python's os.path.realpath() function which changed behavior in Python 3.8.0
with regard to expansion of substitute drives. The changes mentioned above rely
on the newer Python behavior.
- clang/test/Lexer/case-insensitive-include-absolute.c
- clang/test/Lexer/case-insensitive-include-win.c

This change updates the LLVM Getting Started guide to note this newer Python
version dependency for this relatively niche case. Python 3.6.0 remains the
minimum required Python version otherwise.

Author: tahonermann

llvm/docs/GettingStarted.rst

@@ -295,8 +295,9 @@ Package                                                     Version      Notes
 
 .. note::
 
-   #. Only needed if you want to run the automated test suite in the
-      ``llvm/test`` directory.
+   #. Only needed if you want to run the automated test suite. Python 3.8.0
+      or later is needed on Windows if a substitute (virtual) drive is used
+      to access LLVM source code due to ``MAX_PATH`` limitations.
    #. Optional, adds compression / uncompression capabilities to selected LLVM
       tools.
    #. Optional, you can use any other build tool supported by CMake.