Display Greek small letter mu (#14426)

`%time foo()` output is often copied into code comments to explain
performance improvements. The `\xb5` Latin Extended micro sign and the
`\u03bc` Greek small letter mu have different codes but often look
identical.

Output mu to align with:

* [The International System of Units (SI) brochure](
  https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf
  ), such as Table 7 SI prefixes
* NFKC normalized [Python code](https://peps.python.org/pep-3131/
  ) and [domain names](https://unicode.org/reports/tr36/). For example:
```sh
python -c 'print("""class C:
    \xb5=1
print(hex(ord(dir(C)[-1])))""")' | tee /dev/fd/2 | python -
```
```python
class C:
    µ=1
print(hex(ord(dir(C)[-1])))
```
`0x3bc`
* Section 2.5 Duplicated Characters of [Unicode Technical Report 25](
  https://www.unicode.org/reports/tr25/)
> ...U+03BC μ is the preferred character in a Unicode context.
* Ruff confusable mapping [updates](
  https://github.com/astral-sh/ruff/pull/4430/files
  ), currently in the "preview" stage

Add a unit test for UTF-8 display and
https://bugs.launchpad.net/ipython/+bug/348466 ASCII fallback.

IPython/core/magics/execution.py

@@ -1593,16 +1593,16 b' def _format_time(timespan, precision=3):'
         return " ".join(time)
 
     
-    # Unfortunately the unicode 'micro' symbol can cause problems in
+    # Unfortunately characters outside of range(128) can cause problems in
     # certain terminals.  
     # See bug: https://bugs.launchpad.net/ipython/+bug/348466
     # Try to prevent crashes by being more secure than it needs to
     # E.g. eclipse is able to print a µ, but has no sys.stdout.encoding set.
-    units = [u"s", u"ms",u'us',"ns"] # the save value   
-    if hasattr(sys.stdout, 'encoding') and sys.stdout.encoding:
+    units = ["s", "ms", "us", "ns"]  # the safe value
+    if hasattr(sys.stdout, "encoding") and sys.stdout.encoding:
         try:
-            u'\xb5'.encode(sys.stdout.encoding)
-            units = [u"s", u"ms",u'\xb5s',"ns"]
+            "μ".encode(sys.stdout.encoding)
+            units = ["s", "ms", "μs", "ns"]
         except:
             pass
     scaling = [1, 1e3, 1e6, 1e9]

IPython/core/tests/test_magic.py

@@ -530,6 +530,14 b' def test_time_local_ns():'
     del ip.user_ns["myvar"]
 
 
+def test_time_microseconds_display():
+    """Ensure ASCII is used when necessary"""
+    with mock.patch("sys.stdout", io.TextIOWrapper(StringIO(), encoding="utf-8")):
+        assert execution._format_time(0.000001) == "1 \u03bcs"
+    with mock.patch("sys.stdout", io.TextIOWrapper(StringIO(), encoding="ascii")):
+        assert execution._format_time(0.000001) == "1 us"
+
+
 # Test %%capture magic. Added to test issue #13926
 def test_capture():
     ip = get_ipython()