Release 2.1.21

[Eric-Arellano]

On the docket:

• "FileNotFoundError: [Errno 2] No such file or directory" in pex "FileNotFoundError: [Errno 2] No such file or directory" in pex #1098
• Unclosed resource warning for /dev/null in PEX teardown. Unclosed resource warning for /dev/null in PEX teardown. #1101
• Remove --sources-directory / --resources-directory distinction. Remove --sources-directory / --resources-directory distinction. #1100
• Invalid requirement, parse error at "'python_v' Invalid requirement, parse error at "'python_v' #940
• Pex skipping pandas activation Pex skipping pandas activation #1017
• Changing vendored versions does not fully clean up previous version Changing vendored versions does not fully clean up previous version #1096
• Pex discards the current interpreter's PATH entry when it is a directory entry. Pex discards the current interpreter's PATH entry when it is a directory entry. #1109

[jsirois]

Broke out remaining issues to Release 2.1.21 in #1111. This release has enough including a few fixes desirable for Pants 2.{0,1}.