BUG: Polynomials now copy properly (#22669)

On line 502, self.symbol.copy() was called, which
causes an AttributeError, since self.symbol is a
string, so it doesn't have a copy() method. To fix
it, I simply removed the copy() and directly assigned
the string.

Xref https://github.com/numpy/numpy/pull/16154

Co-authored-by: Charles Harris <charlesr.harris@gmail.com>

Some of the values in the documentation of lagfit are in single-backticks,
though single backticks usually means that this is a reference to
something else that sphinx tries to resolve. Here I update values that
reference nothing to use double backticks (verbatim), or emphasis.

The meaning of the second argument in the mathematical notation
N(mu, b) for the normal distribution is not consistent. In some
references it is the variance while in others it is the standard
deviation.  Let's avoid the ambiguity by not using the notation.

Fixes #21296

* limit the number of decimals in Polynomial representation

* tests pass

* parenthesize exponential notation in polynomials

* fixed a long line warning

* added polynomial printoptions tests

* polynomial printoptions typo fixed

* made switch to exp notation in polynomial display more natural

* added a test on switching polynomials to exp notation

* fixed linter errors/warnings

* support for nanstr and infstr printoptions in polynomials

* 10^8 threshold for switching to exp notation when displaying polynomials

* merged in PR #21696 fixing issue #21695

* made linter happy

* made some docstring tests pass

* fixed the docs

Co-authored-by: Lev Maximov <lev.maximov@gmail.com>

Adds a symbol attribute to the polynomials from the np.polynomial package to allow the user to control/modify the symbol used to represent the independent variable for a polynomial expression. This attribute corresponds to the variable attribute of the poly1d class from the old np.lib.polynomial module.

Marked as draft for now as it depends on #15666 - all _str* and _repr* methods of ABCPolyBase and derived classes would need to be modified (and tested) to support this change.

Co-authored-by: Warren Weckesser <warren.weckesser@gmail.com>