Update release notes from some previous PRs

[eric-wieser]

Continuing from #146, adding release notes for:

• Don't allow *dop objects to be mutated, and correct the behavior of zero Dop / Sdop #151
• Add a sympy 1.5 run for CircleCI #164
• Replace the broken linear_expand(x, mode=False) #165
• Drop support for python 3.4 #167
• Use a substitution that sympy 1.3.0 understands #169
• Use a substitution that sympy 1.3.0 understands #170
• Add galgebra.__version__ for easy version inspection #172
• Remove unused flatten_one_level function #175
• Add tests for Dop and fix bugs found #177
• Forbid passing coefficients to odd Mvs #180

https://external-builds.readthedocs.io/html/galgebra/186/changelog.html

[codecov]

Codecov Report

Merging #186 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #186   +/-   ##
=======================================
  Coverage   68.61%   68.61%           
=======================================
  Files           9        9           
  Lines        4684     4684           
=======================================
  Hits         3214     3214           
  Misses       1470     1470

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