unicode bold for symbolic-sized matrices

[cbm755]

New upstream sympy allows bold matrix symbols using bold unicode characters. Example:

>> syms a n 
>> A = sym('A', [n n])
A = (sym) 𝐀  (n×n matrix expression)

>> 2*a*A
ans = (sym) 2⋅a⋅𝐀

This is off by default upstream. I'm not sure how well this is supported by fonts. But here's a possible patch if we wanted this:

--- a/inst/private/python_header.py
+++ b/inst/private/python_header.py
@@ -157,7 +157,7 @@ try:
             except:
                 # e.g., SymPy issue #10414
                 pretty_ascii = str(x)
-            pretty_unicode = sp.pretty(x, use_unicode=True)
+            pretty_unicode = sp.pretty(x, use_unicode=True, mat_symbol_style="bold")
             a = ET.SubElement(et, "item")
             f = ET.SubElement(a, "f")
             f.text = str(OCTCODE_SYM)