Repository: scipy/scipy

Issue:
BUG: Missing Constant in Documentation
### Describe your issue.

The table in the documentation that [lists constants](http://scipy.github.io/devdocs/reference/constants.html) is missing the characteristic impedance of a vacuum. This is because the constant is not listed directly in the CODATA 2018 table (this occured in [#10920](https://github.com/scipy/scipy/pull/10920)); however, it has been maintained in scipy since January 2020 (see [#11343](https://github.com/scipy/scipy/pull/11343)). Because the code that handles the documentation of constants [defines obsolete values](https://github.com/scipy/scipy/blob/40f4455eef9bb64e18eb9a6f557fa6fe4a7dd3ca/scipy/constants/_codata.py#L1541) as those not in the CODATA 2018 table, the constant isn't included in the documentation. I'm a little out of my depth of expertise here, but I'm not sure that the characteristic impedance of a vacuum should be counted as an obsolete constant. I'm not really sure why it's not included in the CODATA 2018 table, despite the fact that it is still listed [on NIST's website](https://physics.nist.gov/cgi-bin/cuu/Value?z0) as a fundamental constant. Is it fair to say that this should not be included as an obsolete constant? I'd love to hear from anybody who knows more about this subject and its nuances, thanks!

### Reproducing Code Example

```python
>>> from scipy import constants
>>> constants.physical_constants['characteristic impedance of vacuum']
(376.73031366686166, 'ohm', 5.61366546036269e-08)
>>> 'characteristic impedance of vacuum' in constants._obsolete_constants
True
```


### Error message

```shell
N/A
```


### SciPy/NumPy/Python version information

1.7.3 1.21.5 sys.version_info(major=3, minor=7, micro=11, releaselevel='final', serial=0)


Discussion:
In the two articles (https://physics.nist.gov/cuu/pdf/JPCRD2018CODATA.pdf, https://physics.nist.gov/cuu/pdf/RevModPhys.93.025010.pdf) listed by NIST on the 2018 adjustment the characteristic impedance of a vacuum is listed as well.
Ok, I think I found the location where the actual text in the source comes from (https://physics.nist.gov/cuu/Constants/Table/allascii.txt) which does indeed list characteristic impedance of a vacuum. A quick diff (https://www.diffchecker.com/1hgzMrhi) shows that characteristic impedance of a vacuum is the only constant that was dropped when the table was inserted into the source code. This makes me think the correct move is to add it back in to the table, which will fix the obsolete issue and put it back into the documentation. Is the redefinition that currently sets the value still necessary?
Apparently, codata data table was updated sometime between the Sep 26, 2019 ( https://web.archive.org/web/20190926042548/https://physics.nist.gov/cuu/Constants/Table/allascii.txt) and Sep 06, 2020 ( https://web.archive.org/web/20200906102720/https://physics.nist.gov/cuu/Constants/Table/allascii.txt ). So, adding the one line should be sufficient to introduce the constant.

Resolving patch:
diff --git a/scipy/constants/_codata.py b/scipy/constants/_codata.py
--- a/scipy/constants/_codata.py
+++ b/scipy/constants/_codata.py
@@ -1194,6 +1194,7 @@
 Boltzmann constant in eV/K                                  8.617 333 262... e-5     (exact)                  eV K^-1
 Boltzmann constant in Hz/K                                  2.083 661 912... e10     (exact)                  Hz K^-1
 Boltzmann constant in inverse meter per kelvin              69.503 480 04...         (exact)                  m^-1 K^-1
+characteristic impedance of vacuum                          376.730 313 668          0.000 000 057            ohm
 classical electron radius                                   2.817 940 3262 e-15      0.000 000 0013 e-15      m
 Compton wavelength                                          2.426 310 238 67 e-12    0.000 000 000 73 e-12    m
 conductance quantum                                         7.748 091 729... e-5     (exact)                  S
@@ -1717,14 +1718,6 @@ def find(sub: str | None = None, disp: bool = False) -> Any:
     'joule-kilogram relationship': (1 / (c * c), 'kg', 0.0),
     'kilogram-joule relationship': (c * c, 'J', 0.0),
     'hertz-inverse meter relationship': (1 / c, 'm^-1', 0.0),
-
-    # The following derived quantities are no longer exact (CODATA2018):
-    # specify separately
-    'characteristic impedance of vacuum': (
-        sqrt(mu0 / epsilon0), 'ohm',
-        sqrt(mu0 / epsilon0) * 0.5 * (
-            physical_constants['vacuum mag. permeability'][2] / mu0
-            + physical_constants['vacuum electric permittivity'][2] / epsilon0))
 }
 
 # sanity check


Test patch: