Remove base ring attribute for the class ModularFormsRing

[DavidAyotte]

After the implementation of the parent/element framework for the class ModularFormsRing in #31559, the __base_ring attribute was not removed in the code. This ticket aims to remove this attribute and associated method so that the class use the inherited parent base ring method instead.

This ticket is part of #31560.

Depends on #31559

CC: @videlec

Component: modular forms

Keywords: gsoc2021 modular forms ring base ring

Author: David Ayotte

Branch/Commit: 1960959

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/32349

[DavidAyotte]

Branch: u/gh-DavidAyotte/remove_base_ring_attribute

[DavidAyotte]

Last 10 new commits:

1b9332e	moved sage.modular.modform.find_generators.py in sage.rings, attempt at making decrecation work (without success)
68509f9	deprecation, added missing newline
2e78c45	revert to commit 7fdca22
99f8ab5	renamed find_generators.py to ring.py, added deprecation
62a9484	docstring updates, fixed weights_list method for zero element
ed90b5a	fixed deprecation, added deprecation for find_generators and basis_for_modform_space
d489a1f	moved deprecation doctests inside find_generators, fixed docstrings
bf19eec	added example info for GradedModularFormElement class, moved some doctests inside the `__init__` method (for coverage)
2e49907	fix docbuild error
1960959	removed base ring attribute, removed base_ring method

[DavidAyotte]

Commit: 1960959

[DavidAyotte]

comment:4

Only the last commit is pertinent (commit 1960959).

[videlec]

Reviewer: Vincent Delecroix

[vbraun]

Changed branch from u/gh-DavidAyotte/remove_base_ring_attribute to 1960959