cleanup: fix some latent open(path).read() et al calls we previously missed
This pattern was banned by check-code way back in 1b4b82063ce2 (may of
2011), but due to a regular expression rewriting bug in check-code
these particular callsites were never detected. Python 3.7 caught the
bug, which then exposed these errors.
Differential Revision: https://phab.mercurial-scm.org/D2863
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