Work around Windows problem by using dummy pool for jobs=1

[Zac-HD]

Per #435, any codemod which is implemented with a lambda cannot be run on more than one file at time. That's because pickle limitations mean we can't use a process pool, and so this patch ensures that we don't use a process pool for jobs=1 and thus enables a "just run serially" workaround.

This is also something of a performance optimisation, since starting a single process to do all the work is likely to take longer than just doing it in the current process.

[codecov-io]

Codecov Report

Merging #436 (4deb245) into master (df231f3) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   94.30%   94.30%           
=======================================
  Files         232      232           
  Lines       22669    22669           
=======================================
  Hits        21379    21379           
  Misses       1290     1290           

Impacted Files	Coverage Δ
libcst/codemod/_cli.py	28.19% <0.00%> (ø)

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[zsol]

Seems reasonable. Thx for the fix!