Build a Python 3.9 environment by default

[Anthchirp]

and update cctbx to 2021.5

[graeme-winter]

Python 3.9 build worked fine for me on macOS 11.4 using system compilers (the ones in Conda no longer work quite right after an xcode update)

Keeping developer builds at the cutting edge seems like a good idea, especially considering that Python 3.8 is now in security patch only support.

[graeme-winter]

Seems like a timely change

[codecov]

Codecov Report

Merging #1735 (fd2e3a1) into main (d012d35) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head fd2e3a1 differs from pull request most recent head b65261e. Consider uploading reports for the commit b65261e to get more accurate results

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
- Coverage   67.22%   67.22%   -0.01%     
==========================================
  Files         616      616              
  Lines       69014    69014              
  Branches     9617     9617              
==========================================
- Hits        46394    46392       -2     
- Misses      20678    20680       +2     
  Partials     1942     1942